packages: use common overlay structure that vcsserver uses.

pkgs/overlays.nix

@@ -0,0 +1,3 b''
+self: super: {
+
+}

default.nix

@@ -24,12 +24,19 b' args@'
 , doCheck ? false
 , ...
 }:
+let pkgs_ = (import <nixpkgs> {}); in
 
 let
   # Use nixpkgs from args or import them. We use this indirect approach
   # through args to be able to use the name `pkgs` for our customized packages.
   # Otherwise we will end up with an infinite recursion.
-  pkgs = args.pkgs or (import <nixpkgs> { });
+  pkgs = args.pkgs or (import <nixpkgs> {
+    overlays = [
+      (import ./pkgs/overlays.nix)
+    ];
+    inherit (pkgs_)
+      system;
+  });
 
   # Works with the new python-packages, still can fallback to the old
   # variant.

shell.nix

@@ -14,7 +14,6 b' let'
 
   enterprise-ce = import ./default.nix {
     inherit
-      pkgs
       pythonPackages
       doCheck;
   };