CI: Downgrade Emscripten to 3.1.18

Emscripten 3.1.19 and 3.1.20 have a showstopping regression that breaks
calling our main function for the editor build.

(cherry picked from commit b353336037af684c2f1455c5e1f4cd649dbc0064)

.github/workflows/javascript_builds.yml

@@ -6,7 +6,7 @@ env:
   # Only used for the cache key. Increment version to force clean build.
   GODOT_BASE_BRANCH: 3.x
   SCONSFLAGS: verbose=yes warnings=all werror=yes debug_symbols=no
-  EM_VERSION: 3.1.20
+  EM_VERSION: 3.1.18
   EM_CACHE_FOLDER: "emsdk-cache"
 
 concurrency: