Ensure $TMP_BUILD_DIR is actually created.

Previously if mktemp failed in function mk_build_dir, die was not
called.  The check for the exit code was actually checking that the
previous export statement completed successfully, not the call to
mktemp.

Change-Id: I477a2ce75c87c8167883ce7aa342e93c40770e29
Merge "Ensure $TMP_BUILD_DIR is actually created."