[SPARK-13358] [SQL] Retrieve grep path when do benchmark

JIRA: https://issues.apache.org/jira/browse/SPARK-13358

When trying to run a benchmark, I found that on my Ubuntu linux grep is not in /usr/bin/ but /bin/. So wondering if it is better to use which to retrieve grep path.

cc davies

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #11231 from viirya/benchmark-grep-path.