packaging: use common structure that rhodecode-enterprise-ce is using for easier diffs

default.nix

@@ -15,15 +15,13 b' let pkgs_ = (import <nixpkgs> {}); in'
 
 let
 
-  # TODO: Currently we ignore the passed in pkgs, instead we should use it
-  # somehow as a base and apply overlays to it.
-  pkgs = 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

@@ -10,7 +10,6 b' let'
 
   vcsserver = import ./default.nix {
     inherit
-      pkgs
       doCheck;
   };