Build script fails on Windows without Cygwin

[peterjob-oi]

https://github.com/blas-lapack-rs/blis-src/blob/main/blis-src/build.rs

In the build script, we currently have:

fn main() {
    let out_dir = PathBuf::from(env("OUT_DIR").unwrap());
    if env("CARGO_FEATURE_SYSTEM").is_none() {
        ...
        run(Command::new("cp").arg("-R").arg("upstream").arg(&build_dir));
        ...
    }...
}

This doesn't work by default on Windows without installing Cygwin (https://www.cygwin.com/ ) to get cp as cmd has no cp command (powershell calls out to Copy-Item, but things are run without a shell by default). I suggest we change the build script not to rely on this (and maybe add a CI test for building on windows).