[github-actions] Attempt to fix pip3 cython installation for macOS build

Author: nikias

.github/workflows/build.yml

@@ -74,7 +74,9 @@ jobs:
           else
             brew install libtool autoconf automake pkgconfig
           fi
-          pip3 install cython
+          python3 -m venv pyenv
+          source pyenv
+          python3 -m pip install cython
       shell: bash
     - name: fetch libplist
       uses: dawidd6/action-download-artifact@v3