tree-wide: do not use pkgs.extend in nixpkgs

Each invocation of pkgs.extends adds 130MB of allocation to the hydra
evaluator. We are already struggling with the amount of memory nixpkgs
requires.

`pkgs.extend` is a useful escape-hatch, but should be not be used inside
of nixpkgs directly.

pkgs/top-level/stage.nix

@@ -200,6 +200,9 @@ let
       then self
       else import ./stage.nix (args // { overlays = args.overlays ++ extraOverlays; });
 
+    # NOTE: each call to extend causes a full nixpkgs rebuild, adding ~130MB
+    #       of allocations. DO NOT USE THIS IN NIXPKGS.
+    #
     # Extend the package set with a single overlay. This preserves
     # preexisting overlays. Prefer to initialize with the right overlays
     # in one go when calling Nixpkgs, for performance and simplicity.