Once Rust hits >= 1.88, revisit `#[clippy attribute]` in `rpassword.rs`

Revisit the `#[allow(clippy::perf)]` in `src/pam/rpassword.rs` to probably change them to `#[allow(clippy::unbuffered_bytes)]` once all the runners are guaranteed to have 1.88 or higher.

It might also be that this clippy lint evaporates or becomes off-by-default, in which case we can remove the attribute.