This is to fix an issue reported in #6373 where the `cp` would fail if `-Psparkr` was not used in the build

cc dragos pwendell

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

Closes #6379 from shivaram/make-distribution-hotfix and squashes the following commits:

08eb7e4 [Shivaram Venkataraman] Copy SparkR lib if it exists in make-distribution