[NativeAOT-LLVM] Remove explicit WASI SDK downloading from CI

[SingleAccretion]

Once we pick up dotnet/runtime#119253, a few things can be simplified:

1. Remove WASI SDK as a prerequisite for runtime build (still needed to actually use the toolchain).
2. Remove WASI SDK downloading from the CI scripts.
3. Wire up AcquireWasiSdk in the runtime test build targets.