this instance: the repository worktree is *not* named borg.
Cherry pick of 4f1157c into 1.0-maint due to f3efcdb
TODO removed since we already did that after 1.0-maint, but 1.0-maint
will never receive the change.
otherwise it spoils the user's nonces and cache dirs with lots of files.
also: remove all BORG_* env vars from the outer environment
fix get_*_dir tests to use monkeypatch.