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/build.yml

@@ -30,7 +30,7 @@ jobs:
     runs-on: ubuntu-latest
     container: kambi/castle-engine-cloud-builds-tools:cge-unstable
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Package Windows
         run: castle-engine package --os=win64 --cpu=x86_64 --verbose
@@ -82,7 +82,7 @@ jobs:
     runs-on: ubuntu-latest
     needs: [release]
     steps:
-    - uses: actions/checkout@v5
+    - uses: actions/checkout@v6
     - name: Update Release Tag
       if: ${{ github.ref == 'refs/heads/master' }}
       run: |