github: bump the github-dependencies group with 1 update

Bumps the github-dependencies group with 1 update: [actions/checkout](https://github.com/actions/checkout).

- [Release notes](https://github.com/actions/checkout/releases)
- [Commits](actions/checkout@v3...v4)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: github-dependencies
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/nix-update-flake.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Install Nix
         uses: DeterminateSystems/nix-installer-action@bc7b19257469c8029b46f45ac99ecc11156c8b2d
       - name: Update flake.lock