ci: Only check nix files when any nix file changed

.github/workflows/nix-parse.yml

@@ -21,10 +21,14 @@ jobs:
           repos/NixOS/nixpkgs/pulls/${{github.event.number}}/files --paginate \
           | jq '.[] | select(.status != "removed" and (.filename | endswith(".nix"))) | .filename' \
           > "$HOME/changed_files"
+        if [[ -s "$HOME/changed_files" ]]; then
+          echo "CHANGED_FILES=$HOME/changed_files" > "$GITHUB_ENV"
+        fi
     - uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
       with:
         # pull_request_target checks out the base branch by default
         ref: refs/pull/${{ github.event.pull_request.number }}/merge
+      if: ${{ env.CHANGED_FILES && env.CHANGED_FILES != '' }}
     - uses: cachix/install-nix-action@7ac1ec25491415c381d9b62f0657c7a028df52a7 # v24
     - name: Parse all changed or added nix files
       run: |
@@ -33,3 +37,4 @@ jobs:
           out="$(nix-instantiate --parse "$file")" || { echo "$out" && ret=1; }
         done < "$HOME/changed_files"
         exit "$ret"
+      if: ${{ env.CHANGED_FILES && env.CHANGED_FILES != '' }}