fs.mkdtemp should return a Buffer when prefix argument is a Buffer

[`fs.mkdtemp`](https://nodejs.org/api/fs.html#fsmkdtempprefix-options-callback) and friends take a prefix argument, which can be a Buffer encoding a non-UTF8 path. But the APIs always return a string, which can mangle the encoding. So these should return a Buffer in that case. See https://github.com/nodejs/node/pull/58516#discussion_r2160220608.