chore(nix): set default package

Without this `nix run github:uncenter/nixpkgs-track` has to be accompanied with a trailing `#nixpkgs-track`. Thanks for the nifty tool! PS: I use a default overlay to make my projects easier to install, inspired by [papis-nvim][1]. After which you can do ```nix nixpkgs.overlays = [ <the-flake-input>.overlays.default ]; environment.packages = [ pkgs.nixpkgs-track ]; ``` I also [included][2] that in my fork, If it sounds interesting I can open another PR for that as well. [1]: https://github.com/jghauser/papis.nvim/blob/68457df80b1330be656984e81d21755cfbd6952b/README.md#nix [2]: https://github.com/uncenter/nixpkgs-track/commit/b1b706eaf3acab37085fe6b2b779ee929523cb5b