This feels like a very hacky solution - especially inside a flake.
However, other do the same so I guess it is ok:
https://github.com/edolstra/dwarffs/blob/a536d80ed446d5a442f6c7c9b12c662e48fe60ea/flake.nix#L41

This reverts commit 55c64295.

This doesn't work because callPackage forwards them to the package
without checking whether the package wants them. I guess this is how
it is supposed to work but it doesn't help with my problem then.

This doesn't work:
- simple fixpoint doesn't work with callPackage
- my release.nix won't work either because we cannot pass additional
  overlays to the nixpkgs we get as a flake input