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

dashboard/builder/bench.go

@@ -205,6 +205,7 @@ func (b *Builder) executeBenchmark(workpath, hash, benchBin, bench string, procs
 	}
 	metrics = metrics1
 	artifacts = append(artifacts, artifacts1...)
+	ok = true
 	return
 }