Update to latest WASI-SDK and the new `wasm32-wasip2` target

[dicej]

Now that Rust supports the wasm32-wasip2 target, we should be able to stop using my fork of WASI-SDK and instead use the latest upstream release. Also, IIRC, Rust builds with -fPIC by default for wasm32-wasip2, so we might be able to stop using the -Z build-std flag to rustc and drop the dependency on nightly.

[dicej]

This has been addressed by #167, although I haven't tried to get rid of the nightly rustc dependency yet; I'll try that next.

[dicej]

I tried to get rid of the nightly rustc dependency, but hit this:

 error[E0658]: use of unstable library feature `wasip2`
    --> /home/dicej/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/pyo3-0.26.0/src/conversions/std/osstr.rs:26:25
     |
  26 |             let bytes = std::os::wasi::ffi::OsStrExt::as_bytes(self);
     |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Chatting with one of the rustc wasm32-wasip2 maintainers, it sounds like this is a known papercut that they just haven't gotten around to addressing yet. Rather than try to hack around it, I'm going to leave it alone and close this issue as complete; the nightly dependency isn't so onerous as to be worth spending more time on, IMO.