dashboard: disable plan9-386 builder

It hasn't passed in months and now spins, wasting resources.

Updates golang/go#31261
Updates golang/go#29801

Change-Id: Idcf13ae915bad4febb156c5c5d49f07f76cf9d49
Reviewed-on: https://go-review.googlesource.com/c/build/+/172797
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

dashboard/builders.go

@@ -1768,6 +1768,7 @@ func init() {
 		HostType:       "host-plan9-386-gce",
 		MaxAtOnce:      2,
 		numTestHelpers: 1,
+		tryOnly:        true, // disable it for now; Issue 31261, Issue 29801
 		shouldRunDistTest: func(distTestName string, isTry bool) bool {
 			switch distTestName {
 			case "api",

dashboard/builders_test.go

@@ -451,11 +451,12 @@ func TestBuilderConfig(t *testing.T) {
 		{b("darwin-386-10_11@go1.12", "go"), onlyPost},
 		{b("darwin-386-10_11@go1.11", "go"), onlyPost},
 
-		// plan9 only lives at master. We don't support any past releases.
-		{b("plan9-386", "go"), onlyPost},
+		// plan9 only lived at master. We didn't support any past releases.
+		// But it's off for now as it's always failing.
+		{b("plan9-386", "go"), none},  // temporarily disabled
+		{b("plan9-386", "net"), none}, // temporarily disabled
 		{b("plan9-386@go1.11", "go"), none},
 		{b("plan9-386@go1.12", "go"), none},
-		{b("plan9-386", "net"), onlyPost},
 		{b("plan9-386@go1.11", "net"), none},
 		{b("plan9-386@go1.12", "net"), none},
 		{b("plan9-amd64-9front", "go"), onlyPost},