ci/eval/compare: manage the "by: package-maintainer" label
While OfBorg is still adding these, it takes a much longer time to do so
compared to the eval action. Since we're adding rebuild labels, I think
it'd be nice to just do it within the eval action.
# Adds the "11.by: package-maintainer" label if all of the packages directly
# changed are maintained by the PR's author. (https://github.com/NixOS/ofborg/blob/df400f44502d4a4a80fa283d33f2e55a4e43ee90/ofborg/src/tagger.rs#L83-L88)