Scrub PYTHONPATH from virtualenv scripts

BUG=chromium:707456
TEST=Run PYTHONPATH=/usr/lib/python3/dist-packages bin/export_to_gcloud

Change-Id: I20ab158ead3ca3f6921d84c30ced911e685ffc1f
Reviewed-on: https://chromium-review.googlesource.com/465550
Commit-Ready: Allen Li <ayatane@chromium.org>
Tested-by: Allen Li <ayatane@chromium.org>
Reviewed-by: David Riley <davidriley@chromium.org>
diff --git a/scripts/virtualenv_wrapper.py b/scripts/virtualenv_wrapper.py
index b6bcae6..95710b9 100755
--- a/scripts/virtualenv_wrapper.py
+++ b/scripts/virtualenv_wrapper.py
@@ -75,6 +75,7 @@
   """
   new_env_dict = env_dict.copy()
   new_env_dict[_VENV_MARKER] = '1'
+  new_env_dict.pop('PYTHONPATH', None)
   return new_env_dict