home-manager: Allow unfree packages

Some packages are unfree, due to their licenses, e.g. android-studio. To use them one needs to allow unfree packages.

In a home-manager flake this can be done as follows.

In one of your modules add:

{ pkgs, ... }: {
  nixpkgs = {
    config = {
      allowUnfree = true;
      allowUnfreePredicate = (_: true);
    };
  };
}

The allowUnfreePredicate is due to home-manager#2942 (I haven't actually checked that it is necessary)