This moves the parsing of ci/OWNERS into the Nix sandbox. We also get
rid of checking out the nixpkgs repo another time in the reviewers
workflow - we already have everything we need in the eval/compare job.
The creation of owners.txt in this way is only temporary, it should
eventually be moved further, similar to how maintainers.json is
currently migrating to a maintainer map for the whole repo stored on the
target branch as artifact.
3bef0dcf65