nix build is broken

If we're going to support nix builds, we need some way to update the nix config from the pyproject.toml as there's no way I'm going to try and keep the package versions in sync otherwise.

Unfortunately there seem to be a lot of projects which are supposed to do this, many of which seem to be unmaintained and none of which I understand well enough to use.

In the meantime the nix configurations are broken, so we should remove them.