lib: replace var with let

PR-URL: #30396
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/v8_prof_polyfill.js

@@ -77,7 +77,7 @@ try {
 const fd = fs.openSync(logFile, 'r');
 const buf = Buffer.allocUnsafe(4096);
 const dec = new (require('string_decoder').StringDecoder)('utf-8');
-var line = '';
+let line = '';
 
 {
   const message = versionCheck(peekline(), process.versions.v8);
@@ -125,7 +125,7 @@ function versionCheck(firstLine, expected) {
     return 'Unable to read v8-version from log file.';
   }
   // Compare major, minor and build; ignore the patch and candidate fields.
-  for (var i = 0; i < 3; i++)
+  for (let i = 0; i < 3; i++)
     if (curVer[i] !== firstLine[i + 1])
       return 'Testing v8 version different from logging version';
 }