Fix MacOS CI using an available runner

.github/workflows/ci.yml

@@ -98,7 +98,7 @@ jobs:
 
   macos:
     name: Build (macOS, Clang)
-    runs-on: macos-11.0
+    runs-on: macos-10.15
     steps:
       - name: Checkout
         uses: actions/[email protected]