Replace outdated dependencies

[sharifhsn]

The atty crate is unmaintained, so I have changed it to its equivalent is-terminal, which is a simple one-liner.

The ecosystem seems to have embraced rustix as a replacement for interacting with libc directly, so I've chosen to replace it, which is another one-liner.

This is my actual last PR 😅

[sharkdp]

For some reason, CI didn't run on this branch (??) - and I missed that. I reverted the rustix commit on master, since it did not compile on Windows: https://github.com/sharkdp/hexyl/actions/runs/3618076985