fix github actions: install linter : flake8

Author: JFU-NAVA-PBC

.github/workflows/ci.yaml

@@ -25,7 +25,7 @@ jobs:
             cd server
             python -m venv bb2_venv
             . ./bb2_venv/bin/activate
-            pip install pipenv
+            pip install pipenv flake8
             pip install cms-bluebutton-sdk
             pipenv lock
             pipenv install --system --deploy --ignore-pipfile