Make review apps use merged results pipelines

Context

See this slack thread.

Review apps is packaging the source code present in the source branch on an MR. It does NOT use the merged results pipeline code.

The problem

When starting the review app CI/CD pipeline, we are building the container images for GitLab in the https://gitlab.com/gitlab-org/build/CNG-mirror project, which is a multi-project pipeline.

When triggering that new pipeline, the code used to make the container images is the latest commit from the branch, NOT the merge commit created in the initial merged results pipeline.

Goal

We build container images that are based on the merge request pipeline commits.

What's blocking us from doing so

TBD