Provide an overlay to add the opnix package

[graham33]

Thanks for the useful project! I was wondering if there's a reason that the NixOS module doesn't provide an easy-to-use overlay to easily add the package to the default package set on NixOS? I understand it might not make sense to add it to environment.systemPackages by default since not everyone would want it there, and I guess a lot of people are getting opnix via the home-manager module (where it's added to home.packages by default).

For the moment, I just used the package directly from the flake, which works fine:

environment.systemPackages = with pkgs; [
      opnix.packages.${system}.default
    ];

[brizzbuzz]

just haven't gotten around to it, happy to accept a PR!!

[HenryNguyen5]

@graham33 Unless i'm mistaken, isnt it already available to be added to your overlays here?

I could see the module automatically adding the overlay though, so you dont have to both use the module and add the overlay to your nixpkgs