Pirbo messes up in 2711

Merged as part of !2721 (merged)

Context

!2711 (merged) was clearly wrong because incomplete (updating full_opam_repository_ref had more consequences than the one taken into account). No one cared because the test checking that was "allowed to fail"!

Manually testing the MR

scripts/update_opam_repo.sh must return an empty opam_repo.patch

Checklist

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• Provide automatic testing (see the testing guide).
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