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/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 }}