stream: replace var with let

PR-URL: #30379
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

lib/internal/streams/end-of-stream.js

@@ -35,14 +35,14 @@ function eos(stream, opts, callback) {
     if (!stream.writable) onfinish();
   };
 
-  var writableEnded = stream._writableState && stream._writableState.finished;
+  let writableEnded = stream._writableState && stream._writableState.finished;
   const onfinish = () => {
     writable = false;
     writableEnded = true;
     if (!readable) callback.call(stream);
   };
 
-  var readableEnded = stream.readableEnded ||
+  let readableEnded = stream.readableEnded ||
     (stream._readableState && stream._readableState.endEmitted);
   const onend = () => {
     readable = false;