Use macos-latest agent for the GTK build (#360)

`macos-latest` still points to macOS 10.15 right now, which has more agent's capacity, and macOS 11.0 agents are slow and unreliable. At the dame time, we have to keep using macOS 11.0 for the core build to have access to the new `App` lifecycle in the native demo project.

.github/workflows/ci.yml

@@ -47,7 +47,7 @@ jobs:
           ./benchmark.sh
 
   gtk_macos_build:
-    runs-on: macos-11.0
+    runs-on: macos-latest
 
     steps:
       - uses: actions/checkout@v2