gnome3: make 3.20 the default

nixos/tests/gnome3_20.nix nixos/tests/gnome3_18.nix

···
       services.xserver.displayManager.auto.user = "alice";
       services.xserver.desktopManager.gnome3.enable = true;
 
-      environment.gnome3.packageSet = pkgs.gnome3_20;
+      environment.gnome3.packageSet = pkgs.gnome3_18;
 
       virtualisation.memorySize = 512;
     };

pkgs/top-level/all-packages.nix

···
   gnome3_18 = recurseIntoAttrs (callPackage ../desktops/gnome-3/3.18 { });
   gnome3_20 = recurseIntoAttrs (callPackage ../desktops/gnome-3/3.20 { });
 
-  gnome3 = self.gnome3_18 // {
-    shellExtensions = {
-      impatience = callPackage ../desktops/gnome-3/extensions/impatience.nix {};
-      system-monitor = callPackage ../desktops/gnome-3/extensions/system-monitor.nix {};
-      volume-mixer = callPackage ../desktops/gnome-3/extensions/volume-mixer.nix {};
-      workspace-grid = callPackage ../desktops/gnome-3/extensions/workspace-grid.nix {};
-    };
-  };
+  gnome3 = self.gnome3_20;
 
   gnome = recurseIntoAttrs self.gnome2;