Download the appropriate WASI SDK for the CPU architecture

[brettcannon]

Right now we download the x64 version unconditionally, but instead we should do it for the right CPU. I think it requires using TARGETPLATFORM and then doing an if in the RUN.

[brettcannon]

Might be able to get away with just TARGETARCH since our containers are all Linux.

https://github.com/BretFisher/multi-platform-docker-build/blob/main/README.md
https://docs.docker.com/build/building/variables/#multi-platform-build-arguments