CI: use macos-13 (Intel) to avoid pre-existing ARM test failure

Author: ncw

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-latest, macos-latest]
+        os: [ubuntu-latest, macos-13]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4