github/workflows: add macOS 13 build

Apparently it is available in public beta since late April.

ref actions/virtual-environments#6426

.github/workflows/build.yml

@@ -79,6 +79,7 @@ jobs:
         os:
           - "macos-11"
           - "macos-12"
+          - "macos-13"
     steps:
       - uses: actions/checkout@v3