nixos/cups-pdf: fix settings type (#438619)

  instanceSettings = name: {

    freeformType =

      with lib.types;

      attrsOf (

        nullOr (oneOf [

          int

          str

          path

          package

      ]);

        ])

      );

    # override defaults:

    # inject instance name into paths,

    # also avoid conflicts between user names and special dirs