This is neccessary to ensure that generated ninja commands invoke python from
PATH instead of the one from VPython's VirtualEnv. On LUCI builders that replace
system python with VPython using vpython:native-python-wrapper swarming tag, a
new VPython VirtualEnv bubble will be created for each script and contain that
script's individual dependencies.

Based on https://cs.chromium.org/chromium/infra/recipes-py/recipes.py?l=88&rcl=66d9d8c66f10ca37dffa4b6d374c3d802a061b01.

R=iannucci@chromium.org

Bug: 793687
Change-Id: I633d2e0dfeda0097072daba102fb013656aaafa2
Reviewed-on: https://chromium-review.googlesource.com/1120817
Commit-Queue: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>