process: replace vars in per_thread.js

PR-URL: #30385
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@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/per_thread.js

@@ -164,7 +164,7 @@ function wrapProcessMethods(binding) {
   }
 
   function kill(pid, sig) {
-    var err;
+    let err;
 
     // eslint-disable-next-line eqeqeq
     if (pid != (pid | 0)) {