In reply to @alxanyae:matrix.org

If anyone else has this problem, I solved it with the following configuration option:

nix.registry."nixpkgs" = {
    from = { id = "nixpkgs"; type = "indirect"; };
    flake = nixpkgs;
};

mkRegistry = attrs: lib.mapAttrs (_: flake: { inherit flake; }) attrs;
registry = mkRegistry { inherit nixpkgs home-manager nixpkgs-master nur other-things-etc; };

this also works

In reply to @anderscs:matrix.org
Nix should do this automatically. When loading a new source into the store it should not try to substitute derivations dependent on it (with maybe some smart exceptions).

in a flake, how can I access the correct output package from a nixosModule output? e.g. given:

let
  systemDependent = utils.eachDefaultSystem(system: {  packages = {…}; });
in systemDependent // {
  nixosModule = {lib, pkgs, config, ...}:
  let
    pkg = systemDependent.packages.<???>.myPackage;
  in …;
}

how do I query the target system for <???>?

In reply to @tomberek:matrix.org
random: I was playing with the eval cache earlier. Found you can edit the sqlite and trick nix build into creating a symlink for an unrelated drv.