update_chroot: force python2 when running depot_tools code

Since depot_tools doesn't work when python->python3, force that to
python2 until they migrate things.

BUG=chromium:984182
TEST=ensure_bootstrap no longer spews errors when python->python3

Change-Id: I80a378d09badb34935edb748cc159874f5c19098
Reviewed-on: https://chromium-review.googlesource.com/1747176
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
1 file changed