rpm: Ensure correct package versions ordering
Previously package versions were sorted according to packages dict keys but this is not reliable as older versions can be sorted after newer ones.
Prefer to sort package versions according to their build time then as it produces a correct ordering and ensure the HEAD branch alias will target the most recent version of a package.
See https://webapp.staging.swh.network/browse/snapshot/c089c3be905534b7849880f8634d9575c554ea20/releases/?origin_url=rpm://Fedora/packages/qpxtool&snapshot=c089c3be905534b7849880f8634d9575c554ea20 as an example, the HEAD
branch is not targeting the most recent version.