nix-build creates the modified package as expected. Fails when imported to configuration.nix

Question

Nix package build, test, install

I want to modify a nix package and use the modified version on my system, simple right?

The active code builds with nix-build modified.nix & nix-env -i ./result works as expected. No errors 👍️

Active Code

{ pkgs ? import <nixpkgs> { } }:

let
  repo = pkgs.fetchFromGitHub {
    owner = "TheCyberArcher";
    repo = "Respect-My-Internet";
    rev = "feb165acd4d39d61d5c29ad7450b14074f0fb031";
    sha256 = "1hcd6dqjxi7jnrnd6n1h792qb6xwmiv855rfykh6rgzlwpgydrzk";
  };
in

pkgs.opensnitch.overrideAttrs (oldAttrs: {
  preBuild = ''
    mkdir -p $out/etc/opensnitchd/rules/
    mkdir -p $out/etc/opensnitchd/blocklist
    cp -r ${repo}/blocklist/respect-my-internet.txt $out/etc/opensnitchd/blocklist/
    cp -r ${repo}/blocklist/ip/ $out/etc/opensnitchd/
    cp -r ${repo}/regex $out/etc/opensnitchd/
    cp -r ${repo}/rules/*.json $out/etc/opensnitchd/rules/
  '';
})

Next incorporated this file into configuration.nix by adding it as a system package.

 environment.systemPackages = [ 
    (import ./modifed.nix)
  ];

Error

I am met with the error:

A definition for option `environment.systemPackages."[definition 1-entry 1]"' is not of type `package'. Definition values:
       - In `/etc/nixos/host.nix': <function, args: {pkgs?}>

I'm not sure where my problem is, any insights would be greatly appreciated. I ran it through some code AIs and worked through those steps to no avail. The final response was reach out online lol

I was expecting the package to build when nixos-rebuild switch was executed since nix-build and nix-env -i results/ worked as expected.

I've tried to rewrite the nix code a few different ways but nothing seems to be working.

Tested on NixOS 23.11 & 24.05 current running system and a fresh/clean cloud image of NixOS 24.05