CBQE-0: fix python_exe: command not found in run_job_linux.sh

Change-Id: Ic45f90a40e8c4a315bd266d3f281304d6534813f
Reviewed-on: http://review.couchbase.org/38503
Reviewed-by: Aruna Piravi <aruna@couchbase.com>
Tested-by: buildbot <build@couchbase.com>