Builder update

[Colin Walters]

I think the most practical thing is to build version N of self-hosting
systems using version N-1 (in addition to supporting building N with N).
If you are using the latest to build the latest, you can more easily get
into unstable states.

For example, if a new version of the Fedora kernel broke virtio-net, and
you immediately upgrade the build VMs, then the system falls apart and
requires manual system administrator intervention - you can't build a
new fixed kernel.

Presumably version N-1 is more stable, although this is somewhat
obviated by the Fedora kernel maintenance model of keeping all branches
in sync.