`om show` should handle all systems

[srid]

The flake in question is from a private repo, but here's a copy:

{
  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixpkgs-unstable";
    flake-parts.url = "github:hercules-ci/flake-parts";
  };
  outputs = inputs:
    inputs.flake-parts.lib.mkFlake { inherit inputs; } {
      systems = [ "x86_64-linux" ]; # iconv doesn't work on macOS
      perSystem = { pkgs, lib, ... }: {
        formatter = pkgs.nixpkgs-fmt;
        apps.default.program = lib.getExe (pkgs.writeShellApplication {
          // snip fields
        });
        devShells.default = pkgs.mkShell {
          packages = [ pkgs.httrack pkgs.wget ];
        };
      };
    };
}

Notice the systems = [ "x86_64-linux" ] - which means running om show won't show anything. Perhaps it should still show other systems. Maybe spit out an error saying no output for current system, and that the user can run om show . --all-systems to display for all systems? Need to figure out UX.

[srid]

Consider how this would work if IFD is used.