lib: main_thread_only change var to let

PR-URL: #30398
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: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

lib/internal/process/main_thread_only.js

@@ -77,7 +77,7 @@ function wrapPosixCredentialSetters(credentials) {
     if (!Array.isArray(groups)) {
       throw new ERR_INVALID_ARG_TYPE('groups', 'Array', groups);
     }
-    for (var i = 0; i < groups.length; i++) {
+    for (let i = 0; i < groups.length; i++) {
       validateId(groups[i], `groups[${i}]`);
     }
     // Result is 0 on success. A positive integer indicates that the