cmd/toolstash: use "go env GOROOT" instead of runtime.GOROOT()

The GOROOT of whatever tool build toolstash is irrelevant. We want
the goroot of the "go" command we're testing.

Change-Id: Ie7e11c74cb445ea694d88c743dbc239a55d47864
Reviewed-on: https://go-review.googlesource.com/43033
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
1 file changed