dashboard: use snapshots on Plan 9 builders

This change enables snapshots on Plan 9 builders,
so running the tests for each of the golang.org/x
repositories will not require to rebuild Go every time.

Change-Id: I305b713a48c155ebba2c03f39633d17a6fd71e6f
Reviewed-on: https://go-review.googlesource.com/c/build/+/208359
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

dashboard/builders.go

@@ -2379,14 +2379,12 @@ func init() {
 	addBuilder(BuildConfig{
 		Name:              "plan9-arm",
 		HostType:          "host-plan9-arm-0intro",
-		SkipSnapshot:      true,
 		shouldRunDistTest: noTestDir,
 		buildsRepo:        onlyMaster,
 	})
 	addBuilder(BuildConfig{
-		Name:         "plan9-amd64-9front",
-		HostType:     "host-plan9-amd64-0intro",
-		SkipSnapshot: true,
+		Name:     "plan9-amd64-9front",
+		HostType: "host-plan9-amd64-0intro",
 		shouldRunDistTest: func(distTestName string, isTry bool) bool {
 			if !noTestDir(distTestName, isTry) {
 				return false
@@ -2401,9 +2399,8 @@ func init() {
 		buildsRepo: onlyMaster,
 	})
 	addBuilder(BuildConfig{
-		Name:         "plan9-386-0intro",
-		HostType:     "host-plan9-386-0intro",
-		SkipSnapshot: true,
+		Name:     "plan9-386-0intro",
+		HostType: "host-plan9-386-0intro",
 		shouldRunDistTest: func(distTestName string, isTry bool) bool {
 			if !noTestDir(distTestName, isTry) {
 				return false