If TMP environment variable is set to Z:\, TempDir returns Z:.
But Z: refers to current directory on Z:, while Z:\ refers to root
directory on Z:. Adjust TempDir to return Z:\.

Fixes #29291

Change-Id: If04d0c7977a8ac2d9d558307502e81beb68776ef
Reviewed-on: https://go-review.googlesource.com/c/154384
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>