Make gradlew-fdroid behave sanely when called standalone
while fixing #519 (closed) I noticed that calling gradlew-fdroid
outside of fdroid-build, i.e. when not having the env vars set it assumes a strange path for gradle_version_dir (which is tailored to the buildserver usecase.)
We should make sure we either abort in this case or assume sane default paths and not clutter random directories with gradle versions.
Edited by Marcus