Cache only http folder of pip

The wheels folder is filled with the Tryton packages that come from the
repository. So they can not be cached as they are related to the repository
revision.
(grafted from 0ec4f9595cbceb45adab4491f1048f4224761a11)

--HG--
branch : 6.0

.gitlab-scripts/gitlab-ci.yml

@@ -93,7 +93,7 @@ check-dist:
     PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip"
   cache:
     paths:
-      - .cache/pip
+      - .cache/pip/http
   before_script:
     - pip install tox
   script: