Fetch default yml files via different project instead of url

This is a workaround for a bug that showed up after gitlab migration. See issue
infrastructure/issues0#62 for further discussion.

(cherry picked from commit 6faf8c94)

.gitlab-ci.yml

 ---
 include:
-  - "https://gitlab.dune-project.org/core/ci-config/raw/master/config/common/releases/2.6.yml"
-  - "https://gitlab.dune-project.org/core/ci-config/raw/master/jobs/common/releases/2.6.yml"
+  - project: 'core/ci-config'
+    ref: master
+    file: 'config/common/2.6.yml'
+  - project: 'core/ci-config'
+    ref: master
+    file: 'jobs/common/2.6.yml'
 
 before_script:
   - . /duneci/bin/duneci-init-job