This reverts commit fb6d63f3fd.
I really hope this finally fixes#99236: evaluation on Hydra.
This time I really did check basically the same commit on Hydra:
https://hydra.nixos.org/eval/1618011
Right now I don't have energy to find what exactly is wrong in the
commit, and it doesn't seem important in comparison to nixos-unstable
channel being stuck on a commit over one week old.
Currently, LXD always use pkgs.zfs, even if boot.zfs.enableUnstable is set. This
change provides the option to change the LXC, LXD and ZFS packages, and
determines the default ZFS package based on zfs.enableUnstable.
