gclient runhooks often calls scripts that depend on scripts in depot_tools.
Currently bot_update only ensures that depot_tools is present in $PATH, but
otherwise uses whatever depot_tools already specified in $PATH. We don't pin
depot_tools before running a recipe, so using whatever depot_tools in $PATH
is not hermetic.

Insert the path to the pinned depot_tools to the beginning of $PATH

R=iannucci@chromium.org

Bug:

Change-Id: I31d04ba2ec7c409f89d58a51502ce3565e3a02a1
Reviewed-on: https://chromium-review.googlesource.com/481072Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>