CI: 'clone' git repository instead of 'fetch'

While the former is slower, the later might lead to errors such as
"fatal: detected dubious ownership in repository at" which is fixed
GitLab upstream via
https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/3538, but
not yet released.

Closes #191

.gitlab-ci.yml

 variables:
   CONTAINER_REGISTRY: $CI_REGISTRY/georg/mat2-ci-images
+  GIT_STRATEGY: clone
 
 stages:
   - linting