After bug 1651806, we're trying to caution people against running `mach build $A_SPECIFIC_TARGET` because it's not generally supported. `dumbmake` is a piece of infrastructure that attempts to make this use case a little bit more useable, but it was always supposed to be a stopgap. There doesn't seem to be a need for it any more.
Differential Revision: https://phabricator.services.mozilla.com/D83136