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/nightly.yml

@@ -55,7 +55,7 @@ jobs:
     steps:
       # (3) checkout this repository in order to "see" the following custom action
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       # (4) "use" the custom action to retrigger the failed "acceptance job" above
       # NOTE: pass the SOURCE_GITHUB_TOKEN to the custom action because (a) it must have

.github/workflows/workflow-restarter-test.yml

@@ -48,7 +48,7 @@ jobs:
     steps:
       # (3) checkout this repository in order to "see" the following custom action
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       # (4) "use" the custom action to retrigger the failed "acceptance job" above
       # NOTE: pass the SOURCE_GITHUB_TOKEN to the custom action because (a) it must have