That test will install cmd/pack for linux_386; we don't want to change
GOROOT in short mode.

Change-Id: I4b00c578a99779a13c558208bfd4115f8f0513fa
Reviewed-on: https://go-review.googlesource.com/12481
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>