builddisks: Only keep one current disk image
builddisks: Only keep one current disk image

We stopped cleaning them due to me removing code I thought wasn't
used.  Fix that, and at the same time, there was no reason to keep a
previous, so don't.

vrutkovs/vrutkovs.github.io,chergert/gnome-continuous,chergert/gnome-continuous,vrutkovs/vrutkovs.github.io,vrutkovs/vrutkovs.github.io,chergert/gnome-continuous