dashboard: explicitly set timeout scale in {freebsd,plan9}/arm builders

These two builders have GO_TEST_TIMEOUT_SCALE values set in their local
environment. Since those variables affect the builder, and will soon be
needed for golang.org/x test timeout scale, set them explicitly in the
dashboard package. That makes more of the builder configuration visible
and removes the need to maintain local configuration on those builders.

For golang/go#56968.

Change-Id: I206cbddab91c3628f43716e8b05fd52d94fba7f2
Reviewed-on: https://go-review.googlesource.com/c/build/+/455520
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

dashboard/builders.go

@@ -2841,6 +2841,7 @@ func init() {
 		env: []string{
 			"GOARM=7",
 			"CGO_ENABLED=1",
+			"GO_TEST_TIMEOUT_SCALE=8", // from builder's local environment as of 2022-12-06
 		},
 		KnownIssues: []int{52679},
 	})
@@ -2863,6 +2864,9 @@ func init() {
 		distTestAdjust: noTestDirAndNoReboot,
 		buildsRepo:     plan9Default,
 		KnownIssues:    []int{49338},
+		env: []string{
+			"GO_TEST_TIMEOUT_SCALE=3", // from builder's local environment as of 2022-12-06
+		},
 	})
 	addBuilder(BuildConfig{
 		Name:     "plan9-amd64-0intro",