Fix regression in #42 that broke failure messages.

A parenthesis was in the wrong place.

Author: mboes

nixpkgs/nixpkgs.bzl

@@ -230,7 +230,7 @@ Command: {arguments}
 Return code: {return_code}
 Error output:
 {stderr}
-""").format(**outputs)
+""".format(**outputs))
   return result