Use monolithic cache for Gradle User Home

- Do not restore cache when GUH exists
- Include RUNNER_OS in the cache key
- Do not save cache on exact hit
- Only save cache in the final post action
- Log before saving cache

package.json

@@ -27,6 +27,7 @@
     "@actions/cache": "1.0.7",
     "@actions/core": "1.4.0",
     "@actions/exec": "1.1.0",
+    "@actions/github": "5.0.0",
     "@actions/glob": "0.2.0",
     "@actions/http-client": "1.0.11",
     "@actions/tool-cache": "1.7.1",