Use own unix::JoinHandleExt trait

[taiki-e]

The standard library's extension traits are not intended to be implemented for types other than the standard library.
Actually, there are discussions about sealing them: rust-lang/rust#81213, rust-lang/rust#80634
As long as we actually have a crate that depends on it, the standard library won't seal it, but it seems preferable to use our own extension trait anyway.

crossbeam/crossbeam-utils/src/thread.rs

Line 550 in a95c02e

impl<T> JoinHandleExt for ScopedJoinHandle<'_, T> {