Build only for macOS 12 for now

Author: kateinoigakukun

.github/workflows/tests.yml

@@ -8,7 +8,7 @@ jobs:
   test-bot:
     strategy:
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os: [macos-12]
     runs-on: ${{ matrix.os }}
     steps:
       - name: Set up Homebrew