ci: upgrade pip for 3.11 on macOS

Applying same workaround as in
python-poetry/poetry#6586.

.github/workflows/tests.yml

@@ -62,6 +62,11 @@ jobs:
           # Using `timeout` is a safeguard against the Poetry command hanging for some reason.
           timeout 10s poetry run pip --version || rm -rf .venv
 
+      # XXX: https://github.com/pypa/pip/issues/11352 causes random failures -- remove once fixed in a release.
+      - name: Upgrade pip on 3.11 for macOS
+        if: ${{ matrix.python-version == '3.11-dev' && matrix.os == 'macOS' }}
+        run: poetry run pip install git+https://github.com/pypa/pip.git@f8a25921e5c443b07483017b0ffdeb08b9ba2fdf
+
       - name: Install dependencies
         run: poetry install