benchmark: use const instead of var in async_hooks

PR-URL: nodejs#31794
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

benchmark/async_hooks/async-resource-vs-destroy.js

@@ -51,7 +51,7 @@ function buildCurrentResource(getServe) {
   }
 
   function init(asyncId, type, triggerAsyncId, resource) {
-    var cr = executionAsyncResource();
+    const cr = executionAsyncResource();
     if (cr !== null) {
       resource[cls] = cr[cls];
     }