This format for the file can be generated automatically using something like:

  nix-prefetch-github \
    --nix \
    --rev $(git rev-parse HEAD) \
    PrivateStorageio ZKAPAuthorizer > /path/to/zkapauthorizer-repo.nix

run in in a checkout of ZKAPAuthorizer

It knows better than we do about its own dependencies.  Let it control them.