dashboard: fix perf builder
One of these "how it ever worked?.." moments.

LGTM=bradfitz
R=adg, bradfitz
CC=golang-codereviews
https://golang.org/cl/158980045
diff --git a/builder/bench.go b/builder/bench.go
index 78e14f3..fa3849d 100644
--- a/builder/bench.go
+++ b/builder/bench.go
@@ -205,6 +205,7 @@
 	}
 	metrics = metrics1
 	artifacts = append(artifacts, artifacts1...)
+	ok = true
 	return
 }