Explicitly allow cache-write for test invocations

The `gradle-build-action` test workflows need to write cache entries,
even when run on non-default branches. This change add explicit configuration
to set `cache-read-only: false` when cache writing is required.

.github/workflows/integ-test-restore-custom-gradle-home.yml

@@ -30,6 +30,8 @@ jobs:
       uses: ./.github/actions/download-dist
     - name: Setup Gradle
       uses: ./
+      with:
+        cache-read-only: false # For testing, allow writing cache entries on non-default branches
     - name: Build using Gradle wrapper
       working-directory: .github/workflow-samples/groovy-dsl
       run: ./gradlew test --info