NixOS-Config/system/modules/legacy.nix

{config, inputs, lib, ...}: {
  # This will add each flake input as a registry to make nix commands consistent with your flake
  nix.registry = (lib.mapAttrs (_: flake: {inherit flake;})) ((lib.filterAttrs (_: lib.isType "flake")) inputs);

  # This will add your inputs to the system's legacy channels making legacy nix commands consistent
  nix.nixPath = ["/etc/nix/path"];
  environment.etc =
    lib.mapAttrs'
    (name: value: {
      name = "nix/path/${name}";
      value.source = value.flake;
    })
    config.nix.registry;
}