Add GRADLE_ACTIONS_CACHE_USE_VENDORED env var for test configuration

Add env var that forces the vendored caching module when set to
'true', decoupling module selection from the Develocity access key.
Set it centrally in init-integ-test so all tests use the vendored
module by default.

Add a dedicated legacy caching integration test that overrides the
env var to 'false' to verify the legacy module works correctly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/integ-test-legacy-caching.yml

@@ -0,0 +1,66 @@
+name: Test legacy caching module
+
+on:
+  workflow_call:
+    inputs:
+      cache-key-prefix:
+        type: string
+        default: '0'
+      runner-os:
+        type: string
+        default: '["ubuntu-latest"]'
+      skip-dist:
+        type: boolean
+        default: false
+
+env:
+  SKIP_DIST: ${{ inputs.skip-dist }}
+  GRADLE_BUILD_ACTION_CACHE_KEY_PREFIX: legacy-caching-${{ inputs.cache-key-prefix }}
+  GRADLE_ACTIONS_CACHE_USE_VENDORED: false
+
+permissions:
+  contents: read
+
+jobs:
+  legacy-caching-seed-build:
+    strategy:
+      max-parallel: 1
+      fail-fast: false
+      matrix:
+        os: ${{fromJSON(inputs.runner-os)}}
+    runs-on: ${{ matrix.os }}
+    steps:
+    - name: Checkout sources
+      uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+    - name: Initialize integ-test
+      uses: ./.github/actions/init-integ-test
+
+    - name: Setup Gradle
+      uses: ./setup-gradle
+      with:
+        cache-read-only: false # For testing, allow writing cache entries on non-default branches
+    - name: Build kotlin-dsl project
+      working-directory: .github/workflow-samples/kotlin-dsl
+      run: ./gradlew build
+
+  legacy-caching-verify-build:
+    needs: legacy-caching-seed-build
+    strategy:
+      max-parallel: 1
+      fail-fast: false
+      matrix:
+        os: ${{fromJSON(inputs.runner-os)}}
+    runs-on: ${{ matrix.os }}
+    steps:
+    - name: Checkout sources
+      uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+    - name: Initialize integ-test
+      uses: ./.github/actions/init-integ-test
+
+    - name: Setup Gradle
+      uses: ./setup-gradle
+      with:
+        cache-read-only: true
+    - name: Build kotlin-dsl project
+      working-directory: .github/workflow-samples/kotlin-dsl
+      run: ./gradlew build --offline

.github/workflows/suite-integ-test-caching.yml

@@ -45,3 +45,8 @@ jobs:
     uses: ./.github/workflows/integ-test-restore-java-toolchain.yml
     with:
       skip-dist: ${{ inputs.skip-dist }}
+
+  legacy-caching:
+    uses: ./.github/workflows/integ-test-legacy-caching.yml
+    with:
+      skip-dist: ${{ inputs.skip-dist }}