CI: allow pip to install packages system-wide on Homebrew

Author: dnicolodi

.github/workflows/tests.yml

@@ -281,6 +281,7 @@ jobs:
         run: |
           brew install --overwrite python@${{ matrix.python }}
           echo /opt/homebrew/opt/python@${{ matrix.python }}/libexec/bin/ >> $GITHUB_PATH
+          python@${{ matrix.python }} -m pip config break-system-packages true
 
       - name: Install Ninja
         run: brew install ninja