check-presubmit: put cros_utils on PYTHONPATH

We do this already for `cros lint`; make sure it's done here, so mypy
can reason about types more accurately.

BUG=b:308443233
TEST=repo upload

Change-Id: Ia0273404934b0846c4ec63927928a7b39afc50cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/4989912
Commit-Queue: George Burgess <gbiv@chromium.org>
Reviewed-by: Bob Haarman <inglorion@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
1 file changed