Move non-smoke restore-gradle-home tests back to the integ-test suite

Partial revert of the extraction in #1027. The caching smoke test should
answer one question quickly: does a seeded cache let a later build run
--offline? Everything else is integration-level.

smoke-test-restore-gradle-home now has just two jobs: seed the cache, then
verify --offline. The build-cache, no-extracted-cache-entries-restored and
pre-existing-gradle-home jobs move to a restored
integ-test-restore-gradle-home, wired back into suite-integ-test-caching.

The seed-build job is intentionally duplicated across the two workflows.
They use distinct cache keys, since both suites run concurrently and would
otherwise write to the same entry.

pre-existing-gradle-home stays pinned to ubuntu-latest, keeping the reason
recorded: pre-creating ~/.gradle is what stops setup-gradle relocating the
Gradle User Home to D:\a\.gradle on Windows, so the job looks for a cache
entry rooted at a different path than the seed build saved.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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

@@ -36,6 +36,11 @@ jobs:
     with:
       skip-dist: ${{ inputs.skip-dist }}
 
+  restore-gradle-home:
+    uses: ./.github/workflows/integ-test-restore-gradle-home.yml
+    with:
+      skip-dist: ${{ inputs.skip-dist }}
+
   restore-java-toolchain:
     uses: ./.github/workflows/integ-test-restore-java-toolchain.yml
     with: