nixos/ddclient: simplify with lib.getExe

nixos/modules/services/networking/ddclient.nix

···
         inherit StateDirectory;
         Type = "oneshot";
         ExecStartPre = "!${pkgs.writeShellScript "ddclient-prestart" preStart}";
-        ExecStart = "${lib.getBin cfg.package}/bin/ddclient -file /run/${RuntimeDirectory}/ddclient.conf";
+        ExecStart = "${lib.getExe cfg.package} -file /run/${RuntimeDirectory}/ddclient.conf";
       };
     };