Run post submit hooks on vpython3 if vpython is unavailable

If bot doesn't have vpython, run post submit hooks with vpython3.
presubmit_support already checks this too, and it will run those
presubmits regardless of USE_PYTHON3 variable.

Bug: 1411979
Change-Id: Id7edb8a10c39d625154dcc5a131becad3d3ebcbb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4457567
Reviewed-by: Garrett Beaty <gbeaty@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
diff --git a/git_cl.py b/git_cl.py
index 509b1cf..c088c07 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -1514,7 +1514,7 @@
     with gclient_utils.temporary_file() as description_file:
       gclient_utils.FileWrite(description_file, description)
       args.extend(['--description_file', description_file])
-      if not py3_only:
+      if not py3_only and os.getenv('LUCI_OMIT_PYTHON2') != 'true':
         p_py2 = subprocess2.Popen(['vpython', PRESUBMIT_SUPPORT] + args)
       p_py3 = subprocess2.Popen(['vpython3', PRESUBMIT_SUPPORT] + args +
                                 ['--use-python3'])