Test runtime Emscripten builds on presubmit.

[ScottTodd]

Progress on https://github.com/iree-org/iree/issues/9855

• Update Emscripten SDK in our Dockerfile
• Delete the Buildkite wasm runtime job (and prerequisite host install job)
• Add a new emscripten job to ci.yml (GitHub Actions pre/post-submit)

This currently just builds the runtime (and iree-test-deps) using Emscripten. Running tests or building the samples at experimental/web will come later (likely as that code gets promoted to runtime/bindings/ in some way)

I also posted an RFC for this and future work: Promoting IREE web platform / Emscripten builds to "stable"