Fix python scripts in src/build/

build/download_nacl_toolchains.py

-#!/usr/bin/python
+#!/usr/bin/env python
 # Copyright (c) 2011 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Shim to run nacl toolchain download script only if there is a nacl dir."""
 
 import os
 import sys
 
 
@@ skipping 17 matching lines @@
   nacl_build_dir = os.path.join(nacl_dir, 'build')
   download_script = os.path.join(nacl_build_dir, 'download_toolchains.py')
   if not os.path.exists(download_script):
     print "Can't find '%s'" % download_script
     print 'Presumably you are intentionally building without NativeClient.'
     print 'Skipping NativeClient toolchain download.'
     sys.exit(0)
   sys.path.insert(0, nacl_build_dir)
   import download_toolchains
   download_toolchains.Main(args)
+  return 0
 
 
 if __name__ == '__main__':
-  Main(sys.argv[1:])
+  sys.exit(Main(sys.argv[1:]))