Fix buildbot script to deal with random file in nacl_tmp.
Fix buildbot script to deal with random file in nacl_tmp.

BUG=none
R=bradnelson@chromium.org

Review URL: https://codereview.chromium.org/1247983002 .

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client