FR: Ability to customize the location of RJE_UNSAFE_CACHE

[tharakadesilva]

This is to help caching in Gitlab CI.

Gitlab CI only allows caching things that are in the cloned repo. When working with Maven, we do something like -Dmaven.repo.local=$CI_PROJECT_DIR/.m2/repository.

Would be nice to have something like RJE_UNSAFE_CACHE_LOCATION to help configure things so that we can cache the unsafe cache location.

This is mainly for our Renovate jobs that run the repinning.