Run a couple of jobs only on Gitlab.com

What does this MR do and why?

This merge request updates a couple of jobs:

• ensure_job_added
• check_description

To only run on GitLab.com. This is because both job fail when mirroring the component on self-managed instances where unauthorized access is not allowed (i.e. a project's visibility is set to private).

This short-term fix comes as a result of an issue that affects the components below.

• https://gitlab.com/components/secret-detection
• https://gitlab.com/components/sast
• https://gitlab.com/components/code-quality
• https://gitlab.com/components/container-scanning

And fix is also ported to each:

• code-quality!9 (merged)
• sast!11 (merged)
• container-scanning!9 (merged)

Please see this thread for more information.

MR acceptance checklist

I have evaluated this MR against the MR acceptance checklist.