Stop running manual image builds on main pipelines

We are adding the manual image build jobs on all pipelines that run on main, but we should only run them on scheduled pipelines so that we aren't filling up unrelated pipelines with jobs that people shouldn't be running. This way you have to specifically go to a scheduled pipeline (or trigger a new scheduled pipeline) to access the manual jobs and rebuild docker images.

For example, it definitely should not run for review apps: https://gitlab.com/gitlab-org/gitlab-docs/-/pipelines/399257359