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>
diff --git a/dashboard/builders_test.go b/dashboard/builders_test.go
index be73a3f..da69de4 100644
--- a/dashboard/builders_test.go
+++ b/dashboard/builders_test.go
@@ -451,11 +451,12 @@
 		{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},