CI: Pin Python 3.12 on macOS

Author: jserv

.github/workflows/main.yml

@@ -345,6 +345,10 @@ jobs:
              .ci/riscv-toolchain-install.sh
              echo "${{ github.workspace }}/toolchain/bin" >> $GITHUB_PATH
              echo "$(brew --prefix llvm@18)/bin" >> $GITHUB_PATH
+     - name: Set up Python 3.12
+       uses: actions/setup-python@v5
+       with:
+         python-version: '3.12'
      - name: Install compiler
        id: install_cc
        uses: rlalik/setup-cpp-compiler@master