To cleanup Gradle User Home, a Gradle build must be executed.
Newer Gradle versions are able to cleanup the home directories of older versions,
but not vice-versa.
With this change, the latest version of Gradle is automatically provisioned
in order to run Gradle User Home cleanup. This ensures a consistent version of
Gradle is used for cleanup, and fixes#33 where Gradle is not pre-installed on
a custom runner.
169bec5d8b