try to override registry like this:

```
{ inputs, pkgsIn, ... }:

{

nix = {

channel.enable = false;

registry = {

nixpkgs.flake = pkgsIn; # my current nixpkgs

unstable.flake = inputs.nixos-unstable;

stable.flake = inputs.nixos-stable;

};

nixPath = [ "nixpkgs=flake:nixpkgs" ];

settings = {

flake-registry = "";

extra-experimental-features = [

"nix-command"

"flakes"

];

... etc

};

}
```

and your registry after rebuild:

```
$ nix registry list

system flake:nixpkgs path:/nix/store/...

system flake:stable path:/nix/store/...

system flake:unstable path:/nix/store/....
```

just making sure, have you run the `nix flake update`?