there is a 10% chance this works

lib/fs.nix

@@ -5,7 +5,7 @@
     path: let
       dirs = builtins.readDir path;
     in
-      builtins.filter (name: dirs.${name} == "directory") (builtins.attrNames dirs);
+      builtins.filter (type: dirs.${type} == "directory") (builtins.attrNames dirs);
 
   getModuleImports =
     path: let

modules/nixos/core/nix.nix

@@ -1,6 +1,12 @@
-{ lib, ... }:
+{ lib, inputs, ... }:
 
-{
+let
+  importOverlays = builtins.map
+    (overlay: overlay { inherit lib inputs; })
+    (builtins.filter 
+      (file: builtins.match file "*.nix")
+      (lib.filesystem.listFilesRecursive ../../../overlays));
+in {
   nix = {
     settings = {
       auto-optimise-store = true;
@@ -18,5 +24,6 @@
       allowUnfree = true;
     };
     hostPlatform = lib.mkDefault "x86_64-linux";
+    overlays = importOverlays;
   };
 }

overlays/nixpkgs-unstable.nix

@@ -0,0 +1,4 @@
+{ inputs }:
+(final: prev: {
+  unstable = inputs.nixpkgs-unstable.legacyPackages.${prev.system};
+})