cmd/go can sometimes take up to 400s on windows due
to various issues (disk I/O on builders being the
latest cause). Increase the timeout scale to account
for this.

Change-Id: I1fd4964472a70fb0f33cf6ed73298c034b9c1fb0
Reviewed-on: https://go-review.googlesource.com/120762
Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>