ci: use installed pip

Upgrading pip unnecessarily can be disruptive, e.g. when pip 20 breaks
[1].  This makes it more predictable, and a bit faster also.

1: pypa/pip#7620

.github/workflows/main.yml

@@ -122,7 +122,7 @@ jobs:
         python-version: ${{ matrix.python }}
     - name: Install dependencies
       run: |
-        python -m pip install --upgrade pip
+        pip --version
         pip install tox coverage
 
     - name: Test without coverage