This should prevent bot_update from overriding vpython by accident.

R=tandrii@chromium.org

Recipe-Nontrivial-Roll: build
Bug: 789808
Change-Id: Ia574204439588862f29f94a80e978b2ff4b57586
Reviewed-on: https://chromium-review.googlesource.com/828069
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>