Add cache for check-trytond-rng

As the job install rnc2rng, it is better to use cache.
(grafted from 2c61802765a020d826969f3924f4eb127fffb85e)

--HG--
branch : 6.0

.gitlab-ci.yml

@@ -215,6 +215,9 @@ check-doc-trytond:
 
 check-trytond-rng:
   extends: .check
+  cache:
+    paths:
+      - .cache/pip/http
   before_script:
     - pip install rnc2rng
   script: