Install protobuf at required version

Author: dandavison

.github/workflows/build-binaries.yml

@@ -58,6 +58,7 @@ jobs:
       - name: Test wheel
         run: |
           python -m venv .test-venv
+          ./.test-venv/bin/pip install 'protobuf>=3.20'
           ./.test-venv/bin/pip install --no-index --find-links=./dist temporalio
           ./.test-venv/bin/pip install pytest
           ./.test-venv/bin/python -m pytest -k tests/test_activity_hello