Add intel-based mac runner to mac CI.

Author: divark

.github/workflows/macos.yml

@@ -12,7 +12,10 @@ on:
 
 jobs:
   build:
-    runs-on: macos-latest
+    strategy:
+      matrix:
+        os: [macos-latest, macos-13]
+    runs-on: ${{ matrix.os }}
 
     steps:
       - uses: actions/checkout@v2