Test pipeline of ci-flatdeb-builder triggers too many (5) MR pipelines

Background

The pipeline testing changes to ci-flatdeb-builder triggers child pipelines in tests/ci-flatdeb-builder-runtimes. But the job pipeline-tests: [runtimes, test] creates a MR in tests/ci-flatdeb-builder-runtimes and from this MR triggers 5 identical pipelines (for instance see tests/ci-flatdeb-builder-runtimes!78 (closed)) but only monitors the last one. Because these pipeline are quite long, the monitoring pipeline reaches the timeout and fails. And thus, we cannot merge changes to ci-flatdeb-builder. The responsible code seems to be ci-flatdeb-builder@3e2ac792

Reproducibility

How often the issue is hit when repeating the steps to reproduce and changing nothing?

Put the ✅ in the most appropriate entry:

1. ✅ always
2. often, but not always
3. rarely

Impact of bug

Waste gitlab and server resources and blocks the merge of changes.

Outcomes

TBD

Management data

This section is for management only, it should be the last one in the description.

/cc @em @Balasubramanian @sudarshan @wlozano

Phabricator link: https://phabricator.apertis.org/T10790