Bug 1662787 - Provide an environment variable to force running `mach python-test` command with Python 2 r=froydnj,firefox-build-system-reviewers,mhentges

This is only useful for `mach` commands that we want to run with Python 3 by default, but for which running with Python 2 is still useful. We now have one such command: `python-test`.

In `mach`, switch on the presence of the `MACH_PY2` environment variable. We only want to allow this for `python-test`, so do that sanity check in `mach` as well.

Differential Revision: https://phabricator.services.mozilla.com/D89162