dashboard: bump plan9 GCE image

Includes CL 158597 (enable SMP in Plan 9 image)

Updates golang/go#29822

Change-Id: Ic158c5153e79635ff1bc877d5dfb12a98f380d50
Reviewed-on: https://go-review.googlesource.com/c/158541
Reviewed-by: Bryan C. Mills <[email protected]>

Author: bradfitz

dashboard/builders.go

@@ -256,7 +256,7 @@ var Hosts = map[string]*HostConfig{
 		OwnerGithub: "0intro",
 	},
 	"host-plan9-386-gce": &HostConfig{
-		VMImage:            "plan9-386-v7",
+		VMImage:            "plan9-386-v8",
 		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",