CI(pip): Ensure pip only if needed

This is to work around a problem in conda with Python 3.7 -
it brings in wrong version of `setuptools` incompatible with Python 3.7.

.github/workflows/run-tests.yaml

@@ -38,7 +38,7 @@ jobs:
       - name: Install dependencies
         run: |
           python --version
-          python -m ensurepip
+          python -m pip || python -m ensurepip --default-pip --upgrade
           python -m pip install --upgrade pip setuptools wheel
           pip --version
           pip install --upgrade virtualenv "tox >= 2.0, < 4"