Update how WASI toolchains are used in CI and bootstrap

This commit updates how the WASI targets are configured with their
toolchain. Long ago a `config.toml` option of `wasi-root` was added to
enable building with the WASI files produced by wasi-libc. Additionally
for CI testing and release building the Rust toolchain has been using a
hard-coded commit of wasi-libc which is bundled with the release of the
`wasm32-wasip1` target, for example.

Nowadays though the wasi-sdk project, the C/C++ toolchain for WASI, is
the go-to solution for compiling/linking WASI code and contains the
more-or-less official releases of wasi-libc. This commit migrates CI to
using wasi-sdk releases and additionally updates `bootstrap` to
recognize when this is configured. This means that with `$WASI_SDK_PATH`
configured there's no further configuration necessary to get a working
build. Notably this also works better for the new targets of WASI as
well, such as `wasm32-wasip2` and `wasm32-wasip1-threads` where the
wasi-sdk release now has libraries for all targets bundled within it.

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