Explain why a job is skipped in UI

Problem

In https://gitlab.com/gitlab-community/modelops/applied-ml/code-suggestions/ai-assist/-/pipelines/1333499293 all jobs in build phase are skipped. In particular, the job build:ingest-image has no needs dependencies, and I don't see rules in the CI configuration that could grant that.

Job skipping can happen for multiple reasons - If they are configured to be skipped, or when they are interruptible and interrupted.

On the build:ingest-image job page there is no info about the skip reason either. It just says "The job has been skipped", which is non-actionable ( #387476).

Proposal

When GitLab decides to skip the job, record the message with the reason why, pass it to the job page and show it to user.