Comment 7 for bug 2069608

There are some other similar requirements that Launchpad already handles. The way it does it is that buildd-manager passes `archive_purpose: "PRIMARY"` to the buildd when it's building something for either the primary archive or for a PPA that has "Require virtualized builders" disabled, and then the buildd side uses that as an indication that the result is intended to be compatible with the primary archive.