It seems the fix hasn’t exactly worked, and so a new pull request is open nixos/nix-channnel: fix setting up the default channel again by nikstur · Pull Request #264831 · NixOS/nixpkgs · GitHub.
I’ll wait for it to be merged and arrive to unstable. Honestly once NixOS Tapir comes out I’ll just stick to the stable branch and only grab a small selection of packages from unstable.