Bump actions/checkout from 5 to 6 in /.github/actions/build-dart-sass

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]>

Author: dependabot[bot]

.github/actions/build-dart-sass/action.yml

@@ -16,7 +16,7 @@ runs:
       run: node -e 'console.log(`version=${JSON.parse(fs.readFileSync("ext/sass/package.json"))["dependencies"]["sass"]}`)' | tee -a "$GITHUB_OUTPUT"
       shell: bash
 
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         repository: sass/dart-sass
         ref: ${{ steps.version.outputs.version }}