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/actions.yaml

@@ -9,7 +9,7 @@ jobs:
   Build-on-Ubuntu:
     runs-on: [ubuntu-latest]
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Install build deps
         run: |
           sudo apt update
@@ -19,6 +19,6 @@ jobs:
   Build-on-macOS:
     runs-on: [macos-latest]
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Build p9p
         run: ./INSTALL