[MINOR] Handle fact that mv is different on linux, mac

Follow up to https://github.com/apache/spark/commit/ae853e8f3bdbd16427e6f1ffade4f63abaf74abb as `mv` throws an error on the Jenkins machines if source and destinations are the same.

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #16302 from shivaram/sparkr-no-mv-fix.