gofix: don't rewrite O_APPEND opens

R=r, rog
CC=golang-dev
https://golang.org/cl/4364041

generate_cert.go

@@ -59,7 +59,7 @@ func main() {
 	certOut.Close()
 	log.Print("written cert.pem\n")
 
-	keyOut, err := os.OpenFile("key.pem", os.O_WRONLY|os.O_CREAT, 0600)
+	keyOut, err := os.OpenFile("key.pem", os.O_WRONLY|os.O_CREAT|os.O_TRUNC, 0600)
 	if err != nil {
 		log.Print("failed to open key.pem for writing:", err)
 		return