fs: change var to let

PR-URL: #30318
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

lib/fs.js

@@ -1408,7 +1408,7 @@ if (isWindows) {
   };
 } else {
   splitRoot = function splitRoot(str) {
-    for (var i = 0; i < str.length; ++i) {
+    for (let i = 0; i < str.length; ++i) {
       if (str.charCodeAt(i) !== CHAR_FORWARD_SLASH)
         return str.slice(0, i);
     }