nixos/gpaste: return sessionPath

GPaste ships keybindings for gnome-control-center. Those depend on GSettings schemas
but there is currently no mechanism for loading schemas other than using global ones
from $XDG_DATA_DIRS. Eventually, I want to add such mechanism but until then,
let's return the impure sessionPath option that was removed in
https://github.com/NixOS/nixpkgs/commit/f63d94eba340c05cd241c478deb518204514fce8

nixos/modules/programs/gpaste.nix

@@ -30,5 +30,7 @@ with lib;
     environment.systemPackages = [ pkgs.gnome3.gpaste ];
     services.dbus.packages = [ pkgs.gnome3.gpaste ];
     systemd.packages = [ pkgs.gnome3.gpaste ];
+    # gnome-control-center crashes in Keyboard Shortcuts pane without the GSettings schemas.
+    services.xserver.desktopManager.gnome3.sessionPath = [ pkgs.gnome3.gpaste ];
   };
 }