Bug 1512188 - Collect environment variables into VCS options; r=tomprince,dustin

This makes behavior consistent across all VCS checkouts. I'm still not
a fan of using environment variables here. But at least this gets us
1 step closer to being able to plug alternate logic in without having
to update use of environment variables outside a single function.

Differential Revision: https://phabricator.services.mozilla.com/D13816