Make artifact output opt-in
The artifacts take up a lot of space and that makes @foutrelis concerned. If would be desirable to make artifact output opt-in.
Projects like archlinux/archiso> don't really need to retain them, having only the metrics should be fine there.
archlinux/releng> which does need the artifacts could pass some option to build_in_archiso_vm.sh
to indicate that they should be output.