CI: Upgrade Emscripten to 2.0.15 (same as official standard builds)
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
(cherry picked from commit 9571ae3a3390958a721d89a20fd6c42fb4cad7a1)
Rémi Verschelde