dashboard: update plan9 builder to v7

I built and tested the v7 image from CL 152037 per the instructions in
env/plan9-386/README.

Change-Id: Ia4dcb1ac8429ab3e54e8ed21719d3ad894529927
Reviewed-on: https://go-review.googlesource.com/c/152238
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/dashboard/builders.go b/dashboard/builders.go
index 71fb8d6..45f5729 100644
--- a/dashboard/builders.go
+++ b/dashboard/builders.go
@@ -256,7 +256,7 @@
 		OwnerGithub: "0intro",
 	},
 	"host-plan9-386-gce": &HostConfig{
-		VMImage:            "plan9-386-v6",
+		VMImage:            "plan9-386-v7",
 		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",