dashboard: set GO_TEST_TIMEOUT_SCALE=2 on plan9-386

On the plan9-386 builder, the cmd/go tests sometimes
take a little more than 3 minutes to complete.
We set GO_TEST_TIMEOUT_SCALE=2, so the cmd/go tests
doesn't time out anymore.

Updates golang/go#18047

Change-Id: I16ce26917ee785ebed8ad16e0370d575a0fdd638
Reviewed-on: https://go-review.googlesource.com/33774
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed