test: replace var with let in pre_execution.js

PR-URL: #30411
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/bootstrap/pre_execution.js

@@ -372,7 +372,7 @@ function initializePolicy() {
       const realIntegrities = new Map();
       const integrityEntries = SRI.parse(experimentalPolicyIntegrity);
       let foundMatch = false;
-      for (var i = 0; i < integrityEntries.length; i++) {
+      for (let i = 0; i < integrityEntries.length; i++) {
         const {
           algorithm,
           value: expected