Add the setup to the flake.nix

In the flake.nix file, add the following:

# ...
  outputs = { self, nixpkgs, nxc }:
    let
      system = "x86_64-linux";
    in {
      packages.${system} = nxc.lib.compose {
        inherit nixpkgs system;
        composition = ./composition.nix;

        # Defines the setup
        setup = ./setup.toml;

      };

      defaultPackage.${system} =
        self.packages.${system}."composition::nixos-test";

      devShell.${system} = nxc.devShells.${system}.nxcShellFull;
    };
}