dashboard: change "plan9" SlowBot alias to refer to plan9-arm

The plan9-arm builder is currently passing fairly reliably on the
dashboard, whereas the plan9-386-0intro builder is frequently failing
with an "unexpected fault address".

Until the plan9-386-0intro build is fixed, the plan9-arm builder seems
more useful as a SlowBot to check architecture-agnostic changes that
affect plan9.

Updates golang/go#35456
Updates golang/go#22227

Change-Id: Ib5ecddafec91ca9274f9f0e15cb98621b41dda85
Reviewed-on: https://go-review.googlesource.com/c/build/+/233797
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

dashboard/builders.go

@@ -83,7 +83,7 @@ var slowBotAliases = map[string]string{
 	"openbsd":        "openbsd-amd64-64",
 	"openbsd-386":    "openbsd-386-64",
 	"openbsd-amd64":  "openbsd-amd64-64",
-	"plan9":          "plan9-386-0intro",
+	"plan9":          "plan9-arm",
 	"plan9-386":      "plan9-386-0intro",
 	"plan9-amd64":    "plan9-amd64-9front",
 	"ppc64":          "linux-ppc64-buildlet",