cluster: replace vars in child.js

PR-URL: #30383
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Beth Griggs <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Denys Otrishko <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>

lib/internal/cluster/child.js

@@ -128,7 +128,7 @@ function rr(message, indexesKey, cb) {
   if (message.errno)
     return cb(message.errno, null);
 
-  var key = message.key;
+  let key = message.key;
 
   function listen(backlog) {
     // TODO(bnoordhuis) Send a message to the master that tells it to