goma_ctl.py recommends that folks run `pip install pywin32`, which will
work up until python in depot_tools is updated, at which point it will
break again.

Running with vpython means that the vpython spec which ships with goma
will correctly take effect.

R=brucedawson@chromium.org, tikuta@chromium.org

Bug: 1033106
Change-Id: Icafadf381df782ab30d6cc156419dd7ed424d2bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1962721
Auto-Submit: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>