uucore: fs feature fails to compile on redox

[xtqqczze]

$ cargo clippy -p uucore --features fs --all-targets --target x86_64-unknown-redox
error[E0425]: cannot find function `symlinkat` in module `rustix::fs`
    --> src/uucore/src/lib/features/fs.rs:1250:21
     |
1250 |         rustix::fs::symlinkat(target, rustix::fs::CWD, dest).map_err(Into::into)

#14623 (comment)

[xtqqczze]

@sylvestre, would you be willing to work on this so that we can add CI support for Redox?

[xtqqczze]

Blocked by bytecodealliance/rustix#1638, we could work around this by using libc directly.