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

@@ -40,7 +40,7 @@ jobs:
     runs-on: ubuntu-latest
     container: kambi/castle-engine-cloud-builds-tools:cge-none
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       # Set env CASTLE_ENGINE_PATH following
       # https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#environment-files
       # https://brandur.org/fragments/github-actions-env-vars-in-env-vars
@@ -81,7 +81,7 @@ jobs:
     name: Build Using macOS
     runs-on: macos-15-intel
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Install FPC
         run: brew install fpc
 
@@ -132,7 +132,7 @@ 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: |