os: use const with early return for path

PR-URL: #54959
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jake Yuesong Li <jake.yuesong@gmail.com>
Reviewed-By: Chemi Atlow <chemi@atlow.co.il>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>

Author: trivikr

lib/os.js

@@ -180,12 +180,13 @@ platform[SymbolToPrimitive] = () => process.platform;
  */
 function tmpdir() {
   if (isWindows) {
-    let path = process.env.TEMP ||
+    const path = process.env.TEMP ||
            process.env.TMP ||
            (process.env.SystemRoot || process.env.windir) + '\\temp';
+
     if (path.length > 1 && StringPrototypeEndsWith(path, '\\') &&
         !StringPrototypeEndsWith(path, ':\\'))
-      path = StringPrototypeSlice(path, 0, -1);
+      return StringPrototypeSlice(path, 0, -1);
 
     return path;
   }