dashboard: bump plan9-386-v5 image to v6

plan9-386-v6 is a new plan9-386 image built with the change
done in CL 129098.

I tested it with cmd/debugnewvm on latest Go master commit,
and all stdlib tests passed, including TestVariousDeadlines.
(cmd/... tests didn't finish in time before cmd/debugnewvm
timed out.) Running it 3 more times, there was still one
TestVariousDeadlines failure and 2 successes, so we'll have
to keep watching it.

Updates golang/go#26945.

Change-Id: Ibd1e990e60c3a69a5fddede18d7a121fa319e79d
Reviewed-on: https://go-review.googlesource.com/130137
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/dashboard/builders.go b/dashboard/builders.go
index 0e42078..400be4b 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -216,7 +216,7 @@
 		OwnerGithub: "0intro",
 	},
 	"host-plan9-386-gce": &HostConfig{
-		VMImage:            "plan9-386-v5",
+		VMImage:            "plan9-386-v6",
 		Notes:              "Plan 9 from 0intro; GCE VM is built from script in build/env/plan9-386",
 		buildletURLTmpl:    "http://storage.googleapis.com/$BUCKET/buildlet.plan9-386",
 		goBootstrapURLTmpl: "https://storage.googleapis.com/$BUCKET/gobootstrap-plan9-386.tar.gz",