Cache locally install pip packages, which should speed up pipeline runs in the long term

.gitlab-ci.yml

@@ -17,7 +17,6 @@ run-tox-pipeline:
   services:
     - postgres:15.4
 
-
   # Change pip's cache directory to be inside the project directory since we can
   # only cache local items.
   variables:
@@ -27,14 +26,11 @@ run-tox-pipeline:
     DATABASE_URI_TEST: 'postgresql://nwa:nwa@postgres:5432/gso-test-db'
     PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip"
 
-  # Pip's cache doesn't store the python packages
-  # https://pip.pypa.io/en/stable/topics/caching/
-  #
-  # If you want to also cache the installed packages, you have to install
-  # them in a virtualenv and cache it as well.
   cache:
+    key: tox-virtualenv
     paths:
       - .cache/pip
+      - venv/
 
   before_script:
     - pip install virtualenv