lib: export rustix too

[lucab]

This exports our specific version of rustix. It is part of our
public API, e.g. through OwnedFd usage.

[cgwalters]

Hmm in theory I think we should have fixed this by using the types from https://docs.rs/io-lifetimes/latest/io_lifetimes/ which are just re-exported by rustix.

But...OTOH the consumers of this crate today are themselves going to be using rustix, and I don't think we want multiple versions of rustix in the depchain.

Perhaps what we should do actually is argue for cap-std exposing rustix (under a feature flag?)

[lucab]

This crate and std-cap do use different OwnedFd, see https://docs.rs/cap-std-ext/0.1.1/cap_std_ext/cmdext/trait.CapStdExtCommandExt.html#tymethod.take_fd_n.
So while cap-std may wish to re-export io-lifetimes (but I think they actually want to wait for stdlib, see bytecodealliance/cap-std#192 (comment)), it won't help here as the method above uses a different one. (That's at least my understanding, I started looking at this just this morning as the ostree-rs CI was broken)