[CI] Add x86 based macOS to testing (#636)

* Use the 'macos-latest' runs-on option which now defaults to 'macos-14' which are
  Apple silicon runners.
   - c.f. https://github.com/actions/runner-images/blob/d31241d2c8c646cd0ba18579836636e62c38f1c9/images/macos/macos-14-arm64-Readme.md
* Keep a 'macos-13' runner to continue to test x86 based macOS for the latest
  Python version.
   - c.f. https://github.com/actions/runner-images/blob/d31241d2c8c646cd0ba18579836636e62c38f1c9/images/macos/macos-13-Readme.md

Author: matthewfeickert

.github/workflows/build_tests.yml

@@ -88,11 +88,12 @@ jobs:
       uses: codecov/codecov-action@v3
 
   macos:
-     runs-on: macos-latest
+     runs-on: ${{ matrix.os }}
      if: "!contains(github.event.head_commit.message, 'no ci')"
      strategy:
        max-parallel: 4
        matrix:
+         os: [macos-latest, macos-13]
          python-version: ["3.11"]
 
      steps: