We have a python2 binary available on all platforms already, so use
that instead of relying on the system copy.  This provides consistent
behavior across all platforms and avoids the impending doom when host
distros stop providing python2 packages.

Change-Id: Id48fe343291ba70981a1a65526ba80d1f7ae7d73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2785114Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Brian Ryner <bryner@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>