wasi-common: UNC paths are not handled correctly on windows

The following test executes correctly on a "normal" disk (C:\) on windows, but not on a network shared drive. @peterhuene determined that this is because UNC paths are not handled correctly in cap-std.
https://github.com/bytecodealliance/wasmtime/issues/2642#issuecomment-776293712

`wasmtime main.wasm --dir .`
```
fn main() -> std::io::Result<()> {
let path = std::path::Path::new("dir.tmp");
std::fs::create_dir(path)?;
assert!(path.exists());
Ok(())
}
```