`cbuild` doesn't pull existing image when it should, and CI is broken regarding that

I noticed that while working on !105 (merged): it was rebuilding the bootstrap containers when it actually should have been just pulling the upstream container.

https://gitlab.freedesktop.org/bentiss/ci-templates/-/jobs/11405135#L95 shows that there is a diff between the downstream and the upstyream image, and instead of running skopeo copy it just bails out.

Also, why on earth would that have passed CI??? There is a specific job that tests exactly that. And, yes of course, the new cbuild script doesn't work correctly there: https://gitlab.freedesktop.org/freedesktop/ci-templates/-/jobs/11227182#L258 Note that the package this-package-should-not-exist is not pulled, but we should fail, not paper over it.

Cc: @whot