Problem caused by: https://gerrit.lix.systems/c/lix/+/993
But this is actually nixpkgs doing extremely silly feature detection of
Nix based on version which it should /absolutely not be doing/.
flake-compat is a kind of unhelpful thing to add here since it makes it
impossible to update lix separately from its module when not using
flakes. We would like non-flakes users to not use the flake by using
module.nix directly.