let's try this

.forgejo/install-emacs/action.yml

@@ -0,0 +1,13 @@
+name: 'Set up Emacs'
+description: 'Install a specific Emacs version for use in your workflow.'
+author: 'Steve Purcell'
+inputs:
+  version:
+    description: 'The version of Emacs to install, e.g. "24.3", or "snapshot" for a recent development version.'
+runs:
+  using: 'composite'
+  steps:
+    - run: ${{ github.action_path }}/install.sh
+      shell: bash
+      env:
+        INPUT_VERSION: ${{ inputs.version }}

.forgejo/install-emacs/flake.lock

@@ -0,0 +1,42 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "locked": {
+        "lastModified": 1667395993,
+        "narHash": "sha256-nuEHfE/LcWyuSWnS8t12N1wc105Qtau+/OdUAjtQ0rA=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "5aed5285a952e0b949eb3ba02c12fa4fcfef535f",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1669165918,
+        "narHash": "sha256-hIVruk2+0wmw/Kfzy11rG3q7ev3VTi/IKVODeHcVjFo=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "3b400a525d92e4085e46141ff48cbf89fd89739e",
+        "type": "github"
+      },
+      "original": {
+        "id": "nixpkgs",
+        "ref": "nixpkgs-unstable",
+        "type": "indirect"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

.forgejo/install-emacs/flake.nix

@@ -0,0 +1,21 @@
+{
+  description = "Setup Emacs Action";
+
+  inputs =
+    {
+      nixpkgs.url = "nixpkgs/nixpkgs-unstable";
+      flake-utils.url = "github:numtide/flake-utils";
+    };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+      in
+      {
+        devShell = pkgs.mkShell {
+          buildInputs = with pkgs; [ pkgs.shellcheck ];
+        };
+      }
+    );
+}

.forgejo/install-emacs/install-nix.sh

@@ -0,0 +1,123 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+if nix_path="$(type -p nix)" ; then
+  echo "Aborting: Nix is already installed at ${nix_path}"
+  exit
+fi
+
+if [[ ($OSTYPE =~ linux) && ($INPUT_ENABLE_KVM == 'true') ]]; then
+  enable_kvm() {
+    echo 'KERNEL=="kvm", GROUP="kvm", MODE="0666", OPTIONS+="static_node=kvm"' | sudo tee /etc/udev/rules.d/99-install-nix-action-kvm.rules
+    sudo udevadm control --reload-rules && sudo udevadm trigger --name-match=kvm
+  }
+
+  echo '::group::Enabling KVM support'
+  enable_kvm && echo 'Enabled KVM' || echo 'KVM is not available'
+  echo '::endgroup::'
+fi
+
+# GitHub command to put the following log messages into a group which is collapsed by default
+echo "::group::Installing Nix"
+
+# Create a temporary workdir
+workdir=$(mktemp -d)
+trap 'rm -rf "$workdir"' EXIT
+
+# Configure Nix
+add_config() {
+  echo "$1" >> "$workdir/nix.conf"
+}
+add_config "show-trace = true"
+# Set jobs to number of cores
+add_config "max-jobs = auto"
+if [[ $OSTYPE =~ darwin ]]; then
+  add_config "ssl-cert-file = /etc/ssl/cert.pem"
+fi
+# Allow binary caches for user
+add_config "trusted-users = root ${USER:-}"
+# Add a GitHub access token.
+# Token-less access is subject to lower rate limits.
+if [[ -n "${INPUT_GITHUB_ACCESS_TOKEN:-}" ]]; then
+  echo "::debug::Using the provided github_access_token for github.com"
+  add_config "access-tokens = github.com=$INPUT_GITHUB_ACCESS_TOKEN"
+# Use the default GitHub token if available.
+# Skip this step if running an Enterprise instance. The default token there does not work for github.com.
+elif [[ -n "${GITHUB_TOKEN:-}" && $GITHUB_SERVER_URL == "https://github.com" ]]; then
+  echo "::debug::Using the default GITHUB_TOKEN for github.com"
+  add_config "access-tokens = github.com=$GITHUB_TOKEN"
+else
+  echo "::debug::Continuing without a GitHub access token"
+fi
+# Append extra nix configuration if provided
+if [[ -n "${INPUT_EXTRA_NIX_CONFIG:-}" ]]; then
+  add_config "$INPUT_EXTRA_NIX_CONFIG"
+fi
+if [[ ! $INPUT_EXTRA_NIX_CONFIG =~ "experimental-features" ]]; then
+  add_config "experimental-features = nix-command flakes"
+fi
+# Always allow substituting from the cache, even if the derivation has `allowSubstitutes = false`.
+# This is a CI optimisation to avoid having to download the inputs for already-cached derivations to rebuild trivial text files.
+if [[ ! $INPUT_EXTRA_NIX_CONFIG =~ "always-allow-substitutes" ]]; then
+  add_config "always-allow-substitutes = true"
+fi
+
+# Nix installer flags
+installer_options=(
+  --no-channel-add
+  --darwin-use-unencrypted-nix-store-volume
+  --nix-extra-conf-file "$workdir/nix.conf"
+)
+
+# only use the nix-daemon settings if on darwin (which get ignored) or systemd is supported
+if [[ (! $INPUT_INSTALL_OPTIONS =~ "--no-daemon") && ($OSTYPE =~ darwin || -e /run/systemd/system) ]]; then
+  installer_options+=(
+    --daemon
+    --daemon-user-count "$(python3 -c 'import multiprocessing as mp; print(mp.cpu_count() * 2)')"
+  )
+else
+  # "fix" the following error when running nix*
+  # error: the group 'nixbld' specified in 'build-users-group' does not exist
+  add_config "build-users-group ="
+  sudo mkdir -p /etc/nix
+  sudo chmod 0755 /etc/nix
+  sudo cp "$workdir/nix.conf" /etc/nix/nix.conf
+fi
+
+if [[ -n "${INPUT_INSTALL_OPTIONS:-}" ]]; then
+  IFS=' ' read -r -a extra_installer_options <<< "$INPUT_INSTALL_OPTIONS"
+  installer_options=("${extra_installer_options[@]}" "${installer_options[@]}")
+fi
+
+echo "installer options: ${installer_options[*]}"
+
+# There is --retry-on-errors, but only newer curl versions support that
+curl_retries=5
+while ! curl -sS -o "$workdir/install" -v --fail -L "${INPUT_INSTALL_URL:-https://releases.nixos.org/nix/nix-2.22.1/install}"
+do
+  sleep 1
+  ((curl_retries--))
+  if [[ $curl_retries -le 0 ]]; then
+    echo "curl retries failed" >&2
+    exit 1
+  fi
+done
+
+sh "$workdir/install" "${installer_options[@]}"
+
+# Set paths
+echo "/nix/var/nix/profiles/default/bin" >> "$GITHUB_PATH"
+# new path for nix 2.14
+echo "$HOME/.nix-profile/bin" >> "$GITHUB_PATH"
+
+if [[ -n "${INPUT_NIX_PATH:-}" ]]; then
+  echo "NIX_PATH=${INPUT_NIX_PATH}" >> "$GITHUB_ENV"
+fi
+
+# Set temporary directory (if not already set) to fix https://github.com/cachix/install-nix-action/issues/197
+if [[ -z "${TMPDIR:-}" ]]; then
+  echo "TMPDIR=${RUNNER_TEMP}" >> "$GITHUB_ENV"
+fi
+
+# Close the log message group which was opened above
+echo "::endgroup::"

.forgejo/install-emacs/install.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+
+# Users should use install-nix-action if they want to customise how Nix is installed.
+
+set -euo pipefail
+
+if ! type -p nix &>/dev/null ; then
+    env INPUT_EXTRA_NIX_CONFIG= \
+        INPUT_INSTALL_OPTIONS= \
+        INPUT_INSTALL_URL= \
+        INPUT_NIX_PATH="nixpkgs=channel:nixos-unstable" \
+        INPUT_ENABLE_KVM=true \
+        "$(dirname "$0")"/install-nix.sh
+    # Make the installed Nix immediately available here
+    source "$GITHUB_ENV"
+    while IFS= read -r dir; do PATH="$dir:$PATH"; done < "$GITHUB_PATH"
+fi
+
+echo "::group::Configuring build cache and installing Emacs"
+nix profile install --accept-flake-config "github:purcell/nix-emacs-ci#emacs-${INPUT_VERSION/./-}"
+echo "::endgroup::"

.forgejo/workflows/ci.yml

@@ -18,7 +18,7 @@ jobs:
           node-version: ${{matrix.node}}
 
       - name: Setup emacs
-        uses: https://github.com/actions/jcs090218/setup-emacs@master
+        uses: uses: ./.forgejo/install-emacs
         with:
           version: ${{ matrix.emacs-version }}