add flake module

flake/util.nix

@@ -0,0 +1,20 @@
+{
+  flake,
+  systems,
+  nixpkgs,
+  ...
+}: let
+  inherit
+    (builtins)
+    attrValues
+    ;
+in {
+  forAllSystems = f:
+    nixpkgs.lib.genAttrs systems (system:
+      f system (import nixpkgs {
+        inherit system;
+        allowUnfree = false;
+        allowBroken = false;
+        overlays = attrValues flake.overlays;
+      }));
+}