Since 'python3' might not exist in Windows, this fixes the 'gn'
script for msysgit users.

Recipe-Manual-Change: infra
Bug: None
Change-Id: Ic5ed76963a7cf1819008470079256a6fd572fb0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3103446Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Auto-Submit: Jamie Madill <jmadill@chromium.org>