cluster: replace vars in child.js

PR-URL: #30383
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

lib/internal/cluster/child.js

@@ -130,7 +130,7 @@ 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