Set PIP_CACHE_DIR for job using pip

(grafted from 76d925f088f8e619c6aae411c3385bf8820e2462)

--HG--
branch : 6.0

.gitlab-ci.yml

@@ -207,6 +207,8 @@ check-doc-trytond:
 
 check-trytond-rng:
   extends: .check
+  variables:
+    PIP_CACHE_DIR: "$CI_PROJECT_DIR/.cache/pip"
   cache:
     paths:
       - .cache/pip/http