Bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v4...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/workflows/githubci.yml

@@ -10,8 +10,8 @@ jobs:
       - uses: actions/setup-python@v5
         with:
           python-version: '3.x'
-      - uses: actions/checkout@v4
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
+      - uses: actions/checkout@v6
         with:
           repository: adafruit/ci-arduino
           path: ci