fuzz: replace unsafe libc::dup by rustix::io::dup

[oech3]

https://docs.rs/rustix/latest/rustix/io/fn.dup.html

e.g.

coreutils/fuzz/uufuzz/src/lib.rs

Line 8 in 375fc3c

use libc::{STDERR_FILENO, STDOUT_FILENO, close, dup, dup2, pipe};