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>

pkgs/applications/audio/cadence/default.nix

@@ -96,5 +96,7 @@ mkDerivation rec {
     license = stdenv.lib.licenses.gpl2Plus;
     maintainers = with stdenv.lib.maintainers; [ worldofpeace ];
     platforms = [ "x86_64-linux" ];
+    # Needs QT 5.14
+    broken = true;
   };
 }

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;
   };
 }

pkgs/applications/video/openshot-qt/default.nix

@@ -55,5 +55,8 @@ mkDerivationWith python3Packages.buildPythonApplication rec {
     license = with licenses; gpl3Plus;
     maintainers = with maintainers; [ AndersonTorres ];
     platforms = with platforms; unix;
+    # Cannot use a newer Qt (5.15) version because it requires qtwebkit
+    # and our qtwebkit fails to build with 5.15. 01bcfd3579219d60e5d07df309a000f96b2b658b
+    broken = true;
   };
 }

pkgs/top-level/all-packages.nix

@@ -9543,15 +9543,7 @@ in
     graalvm8-ee
     graalvm11-ee;
 
-  # Cannot use a newer Qt (5.15) version because it requires qtwebkit
+  openshot-qt = libsForQt5.callPackage ../applications/video/openshot-qt { };
-  # and our qtwebkit fails to build with 5.15. 01bcfd3579219d60e5d07df309a000f96b2b658b
-  openshot-qt = (pkgs.extend (final: prev: rec {
-    qt5 = if stdenv.isDarwin then prev.qt5 else prev.qt514;
-    libsForQt5 = if stdenv.isDarwin then prev.libsForQt5 else prev.libsForQt514;
-    pythonInterpreters = prev.pythonInterpreters.override {
-      pkgs = final;
-    };
-  })).libsForQt5.callPackage ../applications/video/openshot-qt { };
 
   openspin = callPackage ../development/compilers/openspin { };
 
@@ -10541,12 +10533,7 @@ in
   python2Packages = python2.pkgs;
   python3Packages = python3.pkgs;
 
-  pythonInterpreters = callPackage ./../development/interpreters/python {
+  pythonInterpreters = callPackage ./../development/interpreters/python { };
-    # Overrides that apply to all Python interpreters and their packages
-    # Generally, this should be avoided.
-    pkgs = pkgs.extend (final: _: {
-    });
-  };
   inherit (pythonInterpreters) python27 python36 python37 python38 python39 python310 python3Minimal pypy27 pypy36;
 
   # Python package sets.
@@ -20227,17 +20214,7 @@ in
 
   bambootracker = libsForQt5.callPackage ../applications/audio/bambootracker { };
 
-  cadence = let
+  cadence = libsForQt5.callPackage ../applications/audio/cadence { };
-    # Use Qt 5.14 consistently
-    pkgs_ = pkgs.extend(_: prev: {
-      pythonInterpreters = prev.pythonInterpreters.override(oldAttrs: {
-        pkgs = oldAttrs.pkgs.extend(_: _: {
-          qt5 = pkgs.qt514;
-          libsForQt5 = pkgs.libsForQt514;
-        });
-      });
-    });
-  in pkgs_.libsForQt514.callPackage ../applications/audio/cadence { };
 
   cheesecutter = callPackage ../applications/audio/cheesecutter { };
 
@@ -23355,19 +23332,8 @@ in
 
   qemu-utils = callPackage ../applications/virtualization/qemu/utils.nix {};
 
-  # Our 3.10 LTS cannot use a newer Qt (5.15) version because it requires qtwebkit
+  qgis-unwrapped = libsForQt5.callPackage ../applications/gis/qgis/unwrapped.nix {
-  # and our qtwebkit fails to build with 5.15. 01bcfd3579219d60e5d07df309a000f96b2b658b
+    withGrass = false;
-  qgis-unwrapped = let
-    pkgs_ = pkgs.extend(_: prev: {
-      pythonInterpreters = prev.pythonInterpreters.override(oldAttrs: {
-        pkgs = oldAttrs.pkgs.extend(_: _: {
-          qt5 = pkgs.qt514;
-          libsForQt5 = pkgs.libsForQt514;
-        });
-      });
-    });
-  in pkgs_.libsForQt514.callPackage ../applications/gis/qgis/unwrapped.nix {
-      withGrass = false;
   };
 
   qgis = callPackage ../applications/gis/qgis { };

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.