test/scripts/generate-gitlab-ci: mimic matrix syntax of jobnames

The job_names are used in metrics (cloudability) to get an overview
of runtimes per "curated_job_name". This "curated_job_name" is deduced by
splitting by ":", which exists when using matrix-jobs.
e.g. https://github.com/osbuild/osbuild-composer/blob/a237447ca06c3f302e41c710a64e86577cfd2537/.gitlab-ci.yml#L111

As we generate the jobs with the script `generate-gitlab-ci`, this
implementation should work to mimic the behavior and have the
same names as if they were matrix-jobs.