common-updater-scripts: handle default.nix without arguments
common-updater-scripts: handle default.nix without arguments

So that the script can be used outside of nixpkgs.
For example, the default.nix recommended by flake-compat does not take arguments.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs