dashboard: set GO_TEST_TIMEOUT_SCALE for linux-s390x-ibm-race builder

This tries to address intermittent test failures by setting
the GO_TEST_TIMEOUT_SCALE=2 for this builder.

Fixes golang/go#60109.

Change-Id: If980795997410ec6d4e927a105d5fc9f1fd8edae
Reviewed-on: https://go-review.googlesource.com/c/build/+/495655
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

dashboard/builders.go

@@ -2989,6 +2989,7 @@ func init() {
 		buildsRepo: func(repo, branch, goBranch string) bool {
 			return repo == "go" && goBranch == "master"
 		},
+		env: []string{"GO_TEST_TIMEOUT_SCALE=2"},
 	})
 	addBuilder(BuildConfig{
 		Name:        "linux-s390x-crosscompile",