fs: ensure consistency for mkdtemp in both fs and fs/promises

Port changes for mkdtemp from lib/fs.js to lib/internal/fs/promise.js,
ensuring consistent behavior.

Refs: #51078
PR-URL: #53776
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: LiviaMedeiros <livia@cirno.name>

lib/internal/fs/promises.js

@@ -1175,15 +1175,8 @@ async function mkdtemp(prefix, options) {
   prefix = getValidatedPath(prefix, 'prefix');
   warnOnNonPortableTemplate(prefix);
 
-  let path;
-  if (typeof prefix === 'string') {
-    path = `${prefix}XXXXXX`;
-  } else {
-    path = Buffer.concat([prefix, Buffer.from('XXXXXX')]);
-  }
-
   return await PromisePrototypeThen(
-    binding.mkdtemp(path, options.encoding, kUsePromises),
+    binding.mkdtemp(prefix, options.encoding, kUsePromises),
     undefined,
     handleErrorFromBinding,
   );