Clone a specific tag of emscripten in CI build

[microbit-matt-hillsdon]

It's probably not prudent to clone the main branch of emscripten in the CI build.

They have releases/tags so we can use that: https://github.com/emscripten-core/emscripten/releases

[microbit-matt-hillsdon]

We could also document the supported version, as we've found that we need a fairly recent version to avoid a warning when invoking a sync (but cannot yield) C function while the main async C function has yielded.