build_packages: force python2 when running goma

The goma codebase is Python 2-only, so force it when running goma_ctl.

BUG=chromium:1007384
TEST=CQ passes

Change-Id: I9f649cb1fd628381c72f17589a61de5295b0419d
Reviewed-on: https://chromium-review.googlesource.com/1822677
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: Alex Klein <saklein@chromium.org>
1 file changed