revert: restore actions/checkout@v4, adding Node.js to runner instead

Will add Node.js to hostPackages in NixOS runner config
to support GitHub/Gitea Actions ecosystem properly.

.gitea/workflows/nix-update.yml

@@ -21,8 +21,10 @@ jobs:
     runs-on: nixos
     steps:
       - name: Checkout repository
-        run: |
+        uses: actions/checkout@v4
-          git clone ${{ secrets.NIX_UPDATE_TOKEN }}@${{ github.server_url }}/${{ github.repository }}.git .
+        with:
+          fetch-depth: 0
+          token: ${{ secrets.NIX_UPDATE_TOKEN }}
 
       - name: Check for available packages to update
         id: check-packages