Nix has a suprising behavior where if the option `extra-foo` is set before `foo`, then setting `foo` overwrites the setting for `extra-foo`. This is reported as https://github.com/NixOS/nix/issues/9487, and will likely not be fixed any time soon.
This works around this issue by always putting `extra-*` settings after non-extra ones in the nixos-generated `/etc/nix.conf`.
