PRESUBMIT.cfg: run presubmit.py checks more cleanly.

Run the presubmit checks the same way that `git cl presubmit` does.

BUG=chromium:1149626
TEST=manual

Change-Id: I0583ce907c846ecdd1c74f7d20e4139ca90b8cde
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/proto/+/2544918
Auto-Submit: LaMont Jones <lamontjones@chromium.org>
Commit-Queue: Sean McAllister <smcallis@google.com>
Reviewed-by: Sean McAllister <smcallis@google.com>
diff --git a/PRESUBMIT.cfg b/PRESUBMIT.cfg
index be13e21..0fa0e02 100644
--- a/PRESUBMIT.cfg
+++ b/PRESUBMIT.cfg
@@ -1,5 +1,5 @@
 [Hook Scripts]
-git_cl_presubmit = presubmit_support.py $(if [[ "${PRESUBMIT_COMMIT}" = pre-submit ]]; then echo --commit; fi)
+git_cl_presubmit = vpython3 $(which presubmit_support.py) $(if [[ "${PRESUBMIT_COMMIT}" = pre-submit ]]; then echo --commit; fi)
 
 [Hook Overrides]
 cros_license_check: true