GitLab: Support CI_JOB_TOKEN

[mfrieling]

For GitLab it would be better to support the CI_JOB_TOKEN (https://docs.gitlab.com/ee/ci/jobs/ci_job_token.html) instead of a personal access token as this is generated by GitLab for the current user with the user's permissions and removes the need of the project owner/maintainer to create a PAT for the project which other users might be able to see.

I tried it but couldn't get it set up correctly to have permissions for pushing to the split repository.