go.tools/dashboard/builder: fix build

TBR=bradfitz
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/151370044

builder/main.go

@@ -128,7 +128,7 @@ func main() {
 		os.Exit(2)
 	}
 	flag.Parse()
-	if len(flag.Args()) == 0 && !*commitOnly {
+	if len(flag.Args()) == 0 && !*commitWatch {
 		flag.Usage()
 	}