Add experimental support for 'cache-write-only'

There may be cases where it a "fresh" cache entry would be beneficial,
for example if the Gradle User Home cache entry grows over time.

This setting would run the build as if no prior cache entry exists.

.github/workflows/integTest-caching-config.yml

@@ -101,3 +101,42 @@ jobs:
       with:
         script: |
           core.setFailed('No build scan detected')
+
+  # Test seed the cache with cache-write-only and verify with cache-read-only
+  seed-build-write-only:
+    env:
+      GRADLE_BUILD_ACTION_CACHE_KEY_PREFIX: ${{github.workflow}}#${{github.run_number}}:${{github.run_attempt}}-write-only-
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+    - name: Checkout sources
+      uses: actions/checkout@v2
+    - name: Setup Gradle
+      uses: ./
+      with:
+        cache-write-only: true
+    - name: Build using Gradle wrapper
+      working-directory: __tests__/samples/groovy-dsl
+      run: ./gradlew test
+
+  verify-write-only-build:
+    env:
+      GRADLE_BUILD_ACTION_CACHE_KEY_PREFIX: ${{github.workflow}}#${{github.run_number}}:${{github.run_attempt}}-write-only-
+    needs: seed-build-write-only
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
+    steps:
+    - name: Checkout sources
+      uses: actions/checkout@v2
+    - name: Setup Gradle
+      uses: ./
+      with:
+        cache-read-only: true
+    - name: Execute Gradle build with --offline
+      working-directory: __tests__/samples/groovy-dsl
+      run: ./gradlew test --offline
+