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/publish.yml

@@ -15,7 +15,7 @@ jobs:
     name: Build source tarball
     runs-on: ubuntu-24.04
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - uses: actions/setup-python@v6
     - run: |
         python -m pip install --upgrade build
@@ -40,7 +40,7 @@ jobs:
         PIP_CONFIG_SETTINGS="build_ext=-j4"
         CFLAGS="-g0"
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Build sdist
       run: pipx run build --sdist
     - name: Build wheels