Bug 1391106 - Support `mach watch` for non-artifact builds....

Bug 1391106 - Support `mach watch` for non-artifact builds. r=firefox-build-system-reviewers,mhentges

I manually compared `$TOPOBJDIR/faster/unified_install_dist_bin` for
an artifact and a non-artifact build, and found only a few
pre-processing differences.  I can't recall why this wasn't supported
initially; I think it used to be that manifests overlapped and that
the interaction with the RecursiveMake backend was more complicated.
But I see no reason why this shouldn't work, and would like to get
some folks to test it in the wild.

Differential Revision: https://phabricator.services.mozilla.com/D110311