let's try this

.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::"