We still use Emscripten 1.39.9 for official Mono builds so ideally we want to test
against an old Emscripten version to ensure we don't break compatibility.
But then google-closure-compiler-linux broke compatibility for us and is not properly
pinned, so we need to use a more recent version for now to fix CI.
Cf. https://github.com/emscripten-core/emsdk/issues/802
9571ae3a33