build: Only keep one cached buildroot around, not two
build: Only keep one cached buildroot around, not two

These take up a substantial amount of space for some reason, even
though all the regular files are hardlinked.

We don't really need two.

vrutkovs/vrutkovs.github.io,chergert/gnome-continuous,GNOME/gnome-ostree,chergert/gnome-continuous,vrutkovs/vrutkovs.github.io,djdeath/gnome-ostree,GNOME/gnome-ostree,chergert/gnome-continuous,GNOME/gnome-ostree,djdeath/gnome-ostree,vrutkovs/vrutkovs.github.io,djdeath/gnome-ostree