Motivation for this change

Currently the nixos module for running tor uses pkgs.tor to run the service. As a user I would like to be able to run the latest version of tor for my relay. I was able to accomplish this by overriding tor in nixpkgs like so:

  nixpkgs.config = {
    packageOverrides = pkgs: let self = pkgs; in with self; rec {
      tor = unstable.pkgs.tor;
    };
  };

But I would like to be able to not overwrite tor in this way and simply tell the service which package I want it to use. This is something many other modules allow. I could then replace the above with:

   services.tor = {
     enable = true;
     package = unstable.pkgs.tor;
   };

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.