ci: bump actions/checkout from 5 to 6

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](https://github.com/actions/checkout/compare/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] <support@github.com>

.github/workflows/update-flake.yml

@@ -34,7 +34,7 @@ jobs:
             echo "email=$id+$name@users.noreply.github.com"
           } >> "$GITHUB_OUTPUT"
       - name: Checkout repository
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           ref: ${{ matrix.branch }}
           token: ${{ steps.app-token.outputs.token || secrets.GITHUB_TOKEN }}