CI: check-packages should use latest OS images instead of build images

Summary

Running check-packages in older backports can sometimes fail due to the older builder images being obsolete. They can fail because verification keys have been rotated and/or mirrors have been removed. See gitlab-org/release/tasks#11805+ for an example. We should instead use the lasted OS images for this kind of testing, so that even old backport builds will continue to work.

Note, we should use the same change for the new check-package-functionality and check-package-availability jobs added in !7802 (merged)