Also, use a random temporary directory rather than os.TempDir.  Defer
removal of existing random temporary directories.

Change-Id: Id7549031cdf78a2bab28c07b6eeff621bdf6e49c
Reviewed-on: https://go-review.googlesource.com/c/146457
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>