lib: replace var with let in loaders.js

PR-URL: #28081
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

lib/internal/bootstrap/loaders.js

@@ -174,7 +174,7 @@ const {
 } = internalBinding('native_module');
 
 NativeModule.map = new Map();
-for (var i = 0; i < moduleIds.length; ++i) {
+for (let i = 0; i < moduleIds.length; ++i) {
   const id = moduleIds[i];
   const mod = new NativeModule(id);
   NativeModule.map.set(id, mod);