builder-fedora: check-repo target may report false positive

[marmarek]

builder-fedora records built packages in text file, and later when it is
present it uses its content, instead of evaluating spec file content. This may
lead to checking old package versions (when repository was updated - especially
the version file, but packages were not built). We still need to record
output packages somehow - because spec file may contain packages definitions,
which aren't actually generated (when %files section is missing). But it needs
some safeguard to detect if that package list is fresh.