move cache cleanup to avoid a race

move cache cleanup to avoid a race

This should fix doc/bugs/standalone_runshell_can_race_and_fail_to_remove___96____126____47__.cache__47__git-annex__47__locales__47____96___dirs
where 2 runshells were running and the second one tried to clean up
LOCPATH while the first one was still populating it.

By moving the cleanup until after LOCPATH is populated, we guarantee
it's populated, so don't need to worry about such a race with another
process populating our same LOCPATH.