Download the appropriate WASI SDK for the CPU architecture

Right now we download the x64 version unconditionally, but instead we should do it for the right CPU. I think it requires [using `TARGETPLATFORM`](https://docs.docker.com/build/building/multi-platform/#cross-compilation) and then doing an [`if` in the `RUN`](https://stackoverflow.com/questions/67596193/building-a-multi-architecture-docker-image-but-dockerfile-requires-different-pa).