there might still be some bugs to work out here.
this produces a workable image, but with some uncertainty
around that swapfile (the first attempt had /swapfile living on a
tmpfs).
it mostly went smooth, though i lost a .ssh key.
probably the best upgrade process is to do most of the heavy work in the
initrd:
write the new nix config, notably, configuring a tmpfs / mount
and moving the previous / to /nix.
then boot and in the initrd, move all the `/nix/nix/...` items
up a level.
