Artifact mozconfigs are not necessarily up-to-date wrt changes to the
nightly mozconfigs, and all in all, shouldn't be much different from
them.
It's just better to use the nightly mozconfigs (or beta on beta, etc.)
and make the mozconfigs themselves handle the few things that need to be
different when the USE_ARTIFACT environment is set (which is now
consistently set by taskcluster)
This does have the side effect of turning builds that actually don't
support artifact builds red when using --artifact on try, instead of
having them silently not be artifact builds as currently happens.
Depends on D21314
Differential Revision: https://phabricator.services.mozilla.com/D21315
--HG--
extra : moz-landing-system : lando