ci: Remove pip upgrade

OpenSSF scorecard does not want us to upgrade to unspecified versions.

Removing the update seems like the simplest solution, as the version
contained in the ubuntu-20.04 base image is recent enough.

.github/workflows/build_and_test.yaml

@@ -56,7 +56,6 @@ jobs:
 
     - name: Install dependencies
       run: |
-          python -m pip install --upgrade pip
           pip install -r tests/integration/requirements.txt
 
     - name: Execute integration tests