This will allow users on unsupported platforms to successfully bypass vpython
via the recognized environment variable.

R=tandrii@chromium.org, vadimsh@chromium.org

Change-Id: Id63019443321d9f63550fc11e7a4ae3f684b17e7
Reviewed-on: https://chromium-review.googlesource.com/927695
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>