x/tools/dashboard/app: ignore freebsd-arm failures

The previous instance of this change was applied to the release branch.
I don't know why the codereview plugin allowed me to do this.
Thankfully we won't be using it for much longer.

TBR=dfc
R=dave
CC=golang-codereviews
https://golang.org/cl/175870044

dashboard/app/build/notify.go

@@ -38,6 +38,7 @@ const (
 var ignoreFailure = map[string]bool{
 	"dragonfly-386":         true,
 	"dragonfly-amd64":       true,
+	"freebsd-arm":           true,
 	"netbsd-amd64-bsiegert": true,
 	"netbsd-arm-rpi":        true,
 	"plan9-amd64-aram":      true,