top-level: simplify `checked` using `lib.all`

  checked =

    (throwIfNot (lib.isList overlays) "The overlays argument to nixpkgs must be a list.")

      (lib.foldr (

        x: throwIfNot (lib.isFunction x) "All overlays passed to nixpkgs must be functions."

      ) lib.id overlays)

      (throwIfNot (lib.all lib.isFunction overlays) "All overlays passed to nixpkgs must be functions.")

      (throwIfNot (lib.isList crossOverlays) "The crossOverlays argument to nixpkgs must be a list.")

      (

        lib.foldr (

          x: throwIfNot (lib.isFunction x) "All crossOverlays passed to nixpkgs must be functions."

        ) lib.id crossOverlays

        throwIfNot (lib.all lib.isFunction crossOverlays) "All crossOverlays passed to nixpkgs must be functions."

      );

  localSystem = lib.systems.elaborate args.localSystem;