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>

dashboard/builders.go

@@ -128,6 +128,7 @@ var Hosts = map[string]*HostConfig{
 		// only use 1, but we hope that 1 will be more powerful
 		// and we'll stop timing out on tests.
 		machineType: "n1-highcpu-4",
+		env:         []string{"GO_TEST_TIMEOUT_SCALE=2"},
 	},
 	"host-windows-gce": &HostConfig{
 		VMImage:            "windows-buildlet-v2",