Fix python scripts in src/build/

build/download_nacl_toolchains.py

Index: build/download_nacl_toolchains.py
diff --git a/build/download_nacl_toolchains.py b/build/download_nacl_toolchains.py
index 1768d805bf28c07e6fd566272017b7ed66c3b08b..f988891a2a54b0a786c3a17dacdb239e819f4b4d 100755
--- a/build/download_nacl_toolchains.py
+++ b/build/download_nacl_toolchains.py
@@ -1,4 +1,4 @@
-#!/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.
@@ -36,7 +36,8 @@ def Main(args):
   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:]))