Switches from `buildconfig.substs` to `bootstrap_toolchain` for the repackage job
Refactors mozpack/dmg to not depend on the environment; Moves path resolution of tools up to `mozbuild.action.{make_dmg|unpack_dmg}`
Update syntax to python 3.7+
Differential Revision: https://phabricator.services.mozilla.com/D163129