I’ve come around to doing it this way too. systems is not automatically supplied as a flake input - you can get such an input like this:

inputs = {
  systems.url = "github:nix-systems/default";
  # ...
};
outputs = { self, nixpkgs, systems }:
  let
    eachSystem = f: lib.genAttrs (import systems) (system: f nixpkgs.legacyPackages.${system});
  in
  {
    # ...
  } 

The handy thing about importing another flake to get a list of four strings is that anyone consuming your flake can override that input in case they want to use a system that isn’t included in your original flake. There is more information at https://github.com/nix-systems/nix-systems