cmd/go: skip TestBuildOutputToDevNull on Plan 9

TestBuildOutputToDevNull was added in CL 16585.
However, copying to /dev/null couldn't work on Plan 9,
because /dev/null is a regular file. Since it's not
different from any other file, the logic in copyFile
couldn't distinguish it from another, already existing,
file, that we wouldn't want to overwrite.

Change-Id: Ie8d353f318fedfc7cfb9541fed00a2397e232592
Reviewed-on: https://go-review.googlesource.com/16691


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>