nixos/buildbot: use `types.port`

nixos/modules/services/continuous-integration/buildbot/master.nix

···
 
       pbPort = lib.mkOption {
         default = 9989;
-        type = lib.types.either lib.types.str lib.types.int;
+        type = lib.types.either lib.types.str lib.types.port;
         example = "'tcp:9990:interface=127.0.0.1'";
         description = ''
           The buildmaster will listen on a TCP port of your choosing