Cannot compile to wasm32-wasi

[Cyborus04]

I was trying to make a crate for getting cap_std::fs::Dirs from wasi's preopened dirs, but hit a bit of a roadblock
It seems even when compiling to wasi, cap-primitives still tries to access std::os::unix

error[E0433]: failed to resolve: could not find `unix` in `os`
  --> cap-primitives\src\rsix\fs\copy_impl.rs:34:18
   |
34 |     use std::os::unix::fs::PermissionsExt;
   |                  ^^^^ could not find `unix` in `os`

+16 other errors

(on nightly)

[sunfishcode]

It appears cap-std's wasi port has fallen a bit behind. I'll work on that.

[sunfishcode]

This needs rust-lang/rust#90835.