ci(macos): use omp enabled homebrew-llvm-clang

Ref
- https://github.com/Rdatatable/data.table/wiki/Installation/370c4629742886190c08de5e199a8ca8d3d86b24
- https://github.com/actions/runner-images/blob/5c04ad10e394b07619bd68a62b3cd7665e7a3cb3/images/macos/macos-14-arm64-Readme.md

.github/workflows/tests.yml

@@ -24,6 +24,11 @@ jobs:
 
     steps:
       - uses: actions/checkout@v4
+      - name: Use llvm-clang on macOS
+        if: ${{ runner.os == 'macOS' }}
+        run: |
+          export CC= "$(brew --prefix llvm@15)/bin/clang"
+          export CXX= "$(brew --prefix llvm@15)/bin/clang++"
       - name: Set up Python ${{ matrix.python-version }}
         uses: conda-incubator/setup-miniconda@v3
         with: