[ci] Make .gitlab-ci.yml portable

Currently the CI config requires that the core/ci-config project exists on the same gitlab server. This prevents the CI from working when pushing the repository to another server. While one may argue, that one also has to push core/ci-config then, it is a very strong restriction to require that the gitlab group core is available.

This can be avoided by using a gitlab link to the included files directing to the dune server, which still allows to select the master branch.