Cache JDK in GRADLE_HOME instead of APP_HOME

[hakanai]

Our build currently downloads the JDK four times because each included build has a different base location.

It would probably make some sense to instead cache the JDK in the shared Gradle home location, but the structure of the cache might want to be thought through before moving it to such a location. e.g., if I switch from one JDK vendor to another without updating the version, at present, it's a cache hit, and maybe it should be a miss if there is a risk of separate projects interfering with each other just by using different JDK vendors..