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.

doc/using/overlays.xml

@@ -28,6 +28,7 @@
    </para>
 
    <para>
+    NOTE: DO NOT USE THIS in nixpkgs.
     Further overlays can be added by calling the <literal>pkgs.extend</literal> or <literal>pkgs.appendOverlays</literal>, although it is often preferable to avoid these functions, because they recompute the Nixpkgs fixpoint, which is somewhat expensive to do.
    </para>
   </section>