fs: change var to let

PR-URL: nodejs#30318
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Trivikram Kamat <[email protected]>
Reviewed-By: David Carlier <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Сковорода Никита Андреевич <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>

lib/fs.js

@@ -1424,7 +1424,7 @@ 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);
     }