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

.github/workflows/javascript_builds.yml

@@ -6,7 +6,7 @@ env:
   GODOT_BASE_BRANCH: 3.x
   SCONSFLAGS: platform=javascript verbose=yes warnings=all werror=yes debug_symbols=no --jobs=2
   SCONS_CACHE_LIMIT: 4096
-  EM_VERSION: 1.39.20
+  EM_VERSION: 2.0.15
   EM_CACHE_FOLDER: 'emsdk-cache'
 
 jobs: