LOS-789: Use pipeline triggers to add multi-project relationships

We need to start making relationships between projects to know where did each pipeline come from. https://gitlab.cern.ch/ai-config-team/ai-tools already has this (see this example):

The initial idea was to add https://docs.gitlab.com/ee/ci/pipelines/multi_project_pipelines.html as in !45 (closed) but as we need to send CI variables to the downstream pipeline on the fly and using https://docs.gitlab.com/ee/ci/pipelines/multi_project_pipelines.html#pass-cicd-variables-to-a-downstream-pipeline-by-using-variable-inheritance makes it unfeasible, we can do what this MR proposes.

Using https://docs.gitlab.com/ee/ci/triggers/index.html#ci-job-token makes the relationship appear in the Pipeline graph view, no need to explicitly create the trigger under https://gitlab.cern.ch/linuxsupport/testing/image-ci/-/settings/ci_cd. The only drawback is that we cannot get rid of the pipeline status checking Alex wrote, as we cannot move to the next stage until the testing concludes.

Check https://gitlab.cern.ch/linuxsupport/koji-image-build/-/pipelines/2931645 for an example of this MR running only for CC7: