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/applications/gis/qgis/unwrapped.nix

@@ -53,5 +53,8 @@ in mkDerivation rec {
     license = lib.licenses.gpl2Plus;
     platforms = with lib.platforms; linux;
     maintainers = with lib.maintainers; [ lsix ];
+    # Our 3.10 LTS cannot use a newer Qt (5.15) version because it requires qtwebkit
+    # and our qtwebkit fails to build with 5.15. 01bcfd3579219d60e5d07df309a000f96b2b658b
+    broken = true;
   };
 }