CI: Fix Emscripten build.

.github/workflows/build.yml

@@ -482,12 +482,12 @@ jobs:
         emsdk-master/emsdk install latest
         emsdk-master/emsdk activate latest
 
-    - name: Build example_emscripten_opengl3
+    - name: Build example_sdl_opengl3 with Emscripten
       run: |
         pushd emsdk-master
         source ./emsdk_env.sh
         popd
-        make -C examples/example_emscripten_opengl3
+        make -C examples/example_sdl_opengl3 -f Makefile.emscripten
 
     - name: Build example_emscripten_wgpu
       run: |