Fix 'cipd - build packages' step for internal waterfall.

build/build.py

 #!/usr/bin/env python
 # Copyright 2015 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.
 
 """This script rebuilds Python & Go universes of infra.git multiverse and
 invokes CIPD client to package and upload chunks of it to the CIPD repository as
 individual packages.
 
 See build/packages/*.yaml for definition of packages.
@@ skipping 160 matching lines @@
   """Returns content of YAML file as python dict."""
   # YAML lib is in venv, not activated here. Go through hoops.
   oneliner = (
       'import json, sys, yaml; '
       'json.dump(yaml.safe_load(sys.stdin), sys.stdout)')
   if sys.platform == 'win32':
     python_venv_path = ('Scripts', 'python.exe')
   else:
     python_venv_path = ('bin', 'python')
   executable = os.path.join(py_venv, *python_venv_path)
+  env = os.environ.copy()
+  env.pop('PYTHONPATH', None)
   proc = subprocess.Popen(
       [executable, '-c', oneliner],
       executable=executable,
       stdin=subprocess.PIPE,
-      stdout=subprocess.PIPE)
+      stdout=subprocess.PIPE,
+      env=env)
   with open(path, 'r') as f:
     out, _ = proc.communicate(f.read())
   if proc.returncode:
     raise BuildException('Failed to parse YAML at %s' % path)
   return json.loads(out)
 
 
 def should_process_on_builder(pkg_def_file, py_venv, builder):
   """Returns True if package should be processed by current CI builder."""
   if not builder:
@@ skipping 311 matching lines @@
       args.build,
       args.upload,
       args.service_url,
       args.tags or [],
       args.service_account_json,
       args.json_output)
 
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv[1:]))