cmd/coordinator: include linux-amd64-kube in staging

Change-Id: Ie27f4db76aa48665c2fce65fffda655382be88a3
Reviewed-on: https://go-review.googlesource.com/18442
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/coordinator/coordinator.go

@@ -360,6 +360,7 @@ func stagingClusterBuilders() map[string]dashboard.BuildConfig {
 	for _, name := range []string{
 		"linux-arm",
 		"linux-amd64",
+		"linux-amd64-kube",
 		"linux-amd64-race",
 		"windows-amd64-gce",
 		"plan9-386",