Bump actions/checkout from 3 to 4 (#20)

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [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/v3...v4)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/build-nightly-images.yml

@@ -37,7 +37,7 @@ jobs:
     steps:
 
     - name: Checkout
-      uses: actions/checkout@v3.5.2
+      uses: actions/checkout@v4
       with:
         clean: false
 

.github/workflows/index.yml

@@ -18,7 +18,7 @@ jobs:
     steps:
 
       - name: Checkout repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           fetch-depth: 1
           repository: armbian/community