Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/update.yaml

@@ -8,7 +8,7 @@ jobs:
   lockfile:
     runs-on: ubuntu-22.04
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: cachix/install-nix-action@v23
         with:
           nix_path: nixpkgs=channel:nixos-unstable