gz-collections.yaml can generate two abichecker jobs

[j-rivero]

Found in #1094.

Depending on the ci_configurations sets the gz-collections.yaml can generate more than one abichecker job to be run on PRs when the same source (repo + branch) is found in two different collections than don't have the same ci_config assigned. i.e: gz-math7 being on Fortress (ciconfig focal) and Garden (ciconfig jammy).

To reduce the workload in the buildfarm, we should prefer to run only one of the jobs.

Side note: modifying the yaml structure for the collections is something I'm trying to avoid since keeping it listing just the software units make it potentially compatible with rosdistro if decide to move the system to use it at some point in the future.

Note: when fixed, remember to re-enable #1099