Change /bin/env to /usr/bin/env in helper python script, as it is more portable

src/etc/2014-06-rewrite-bytes-macros.py

@@ -1,4 +1,4 @@
-#!/bin/env python
+#!/usr/bin/env python
 #
 # Copyright 2014 The Rust Project Developers. See the COPYRIGHT
 # file at the top-level directory of this distribution and at