Python multiprocessing is using sockets to communicate. UNIX socket names are
bound by PATH_MAX which includes temporary directory name.

gsutil uses multiprocessing, which creates file sockets in the tmpdir.
On some testing configurations, the tmpdir is set to a very long path, which
causes gsutil to fail with AF_UNIX path too long errors.

To prevent multiprocess failures, this CL overrides temporary directory
dictated by the environment to /tmp.

Bug: 812581
Change-Id: Idcd99d13b2e20b3095111fa26ec4e242848c8848
Reviewed-on: https://chromium-review.googlesource.com/923170Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>