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.

.github/workflows/web_builds.yml

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