Bump actions/checkout from 5 to 6 (#18)

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/nightly.yml

@@ -28,7 +28,7 @@ jobs:
       json: ${{ steps.fetch-json.outputs.json }}
       revision: ${{ steps.fetch-revision.outputs.revision }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           submodules: true
       - name: Download source
@@ -62,7 +62,7 @@ jobs:
         image: ${{ fromJson(needs.set-up-matrix.outputs.images) }}
     name: ${{ matrix.image.variant }} - ${{ matrix.image.platform }}
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           submodules: true
       - run: echo '${{ needs.set-up-matrix.outputs.json }}' > docker-ruby/versions.json
@@ -191,7 +191,7 @@ jobs:
     if: success() || failure()
 
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
         with:
           submodules: true
       - uses: actions/download-artifact@v6