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](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] <support@github.com>

Author: dependabot[bot]

.github/workflows/build.yml

@@ -15,7 +15,7 @@ jobs:
       commit: ${{ steps.latest_commit.outputs.commit }}
     steps:
     - name: Clone truffleruby
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
       with:
         repository: truffleruby/truffleruby
         path: truffleruby
@@ -50,7 +50,7 @@ jobs:
     outputs:
       tag: ${{ steps.tag.outputs.tag }}
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
       with:
         fetch-depth: 0
       if: github.event_name != 'push'
@@ -83,7 +83,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - name: Clone truffleruby
-      uses: actions/checkout@v5
+      uses: actions/checkout@v6
       with:
         repository: truffleruby/truffleruby
         ref: ${{ needs.prepare.outputs.commit }}