Auto merge of #105018 - zertosh:path_buf_deref_mut, r=dtolnay

Implement DerefMut for PathBuf

Without this, there's no way to get a `&mut Path` from `PathBuf` without
going through `into_boxed_path`. This is relevant now that #105002 adds
`PathBuf::as_mut_os_string` and `Path::as_mut_os_str`.

diff --cc library/std/src/path.rs
Simple merge