ci: do not use default before_script

Executive summary of changes:

For the new security scan job to succeed we have to stop using before_script and use anchors instead (see https://docs.gitlab.com/ci/yaml/yaml_optimization/#yaml-anchors-for-scripts).

Associated issue

resolves #244 (closed)

More details: n/a

To-do list before merging:

• Changelog has been updated