Add custom overlay

Fixes #20

modules/default.nix

@@ -1,5 +1,21 @@
 { config, lib, pkgs, ... }:
-
+let
+  # Taken from https://nixos.wiki/wiki/Overlays
+  overlaysCompat = pkgs.writeTextFile {
+    name = "overlays-compat";
+    destination = "/overlays.nix";
+    text = ''
+      self: super:
+      with super.lib;
+      let
+        # Load the system config and get the `nixpkgs.overlays` option
+        overlays = (import <nixpkgs/nixos> { }).config.nixpkgs.overlays;
+      in
+        # Apply all overlays to the input of the current "main" overlay
+        foldl' (flip extends) (_: super) overlays self
+    '';
+  };
+in
 {
   # Options that affect multiple modules
   options.sbruder = {
@@ -81,6 +97,7 @@
       nixPath = [
         "/var/src" # pinned nixpkgs and configuration
         "nixpkgs=/var/src/nixpkgs" # for nix run
+        "nixpkgs-overlays=${overlaysCompat}"
       ];
       # Make sudoers trusted nix users
       trustedUsers = [ "@wheel" ];
@@ -103,8 +120,13 @@
       packageOverrides = pkgs: {
         unstable = import (import ../nix/sources.nix).nixpkgs-unstable {
           config = config.nixpkgs.config;
+          overlays = config.nixpkgs.overlays;
         };
       };
     };
+
+    nixpkgs.overlays = [
+      (import ../pkgs)
+    ];
   };
 }

pkgs/default.nix

@@ -0,0 +1 @@
+self: super: { }