[recipe_engine] Remove all VirtualEnvs from $PATH.
[recipe_engine] Remove all VirtualEnvs from $PATH.

This will prevent the recipe engine from accidentally leaking its own
VirtualEnv into subprocesses (step calls will respect the remaining value
of $PATH). If recipe engine needs to invoke a subprocesses sharing the
same VirtualEnv, it can use sys.executable.

R=a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org

Bug: 793325, 784597
Change-Id: I03f51900b64f5f7abdd90bf24db48eefa6250564
Reviewed-on: https://chromium-review.googlesource.com/849615
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@chromium.org>
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/recipes-py,luci/recipes-py