gpick: ensure GSettings schemas are available

Running gpick on a system where GSettings schemas are not provided (e.g.
XMonad with no desktop environment), any attempt to save or export color
info from gpick will cause gpick to crash:

GLib-GIO-ERROR **: 19:12:55.095: No GSettings schemas are installed on the system

The nixpkgs manual describes how to fix such errors:
https://nixos.org/manual/nixpkgs/stable/#ssec-gnome-common-issues-no-schemas

Adding wrapGAppsHook to the nativeBuildInputs appears to be all that was
needed to fix the issue in this case.

pkgs/tools/misc/gpick/default.nix

···
-{ stdenv, fetchFromGitHub, cmake, glib, boost, pkg-config, gtk3, ragel, lua, lib }:
+{ stdenv
+, fetchFromGitHub
+, cmake
+, glib
+, wrapGAppsHook
+, boost
+, pkg-config
+, gtk3
+, ragel
+, lua
+, lib
+}:
 
 stdenv.mkDerivation rec {
   pname = "gpick";
···
     sha256 = "sha256-Z67EJRtKJZLoTUtdMttVTLkzTV2F5rKZ96vaothLiFo=";
   };
 
-  nativeBuildInputs = [ cmake pkg-config ];
+  nativeBuildInputs = [ cmake pkg-config wrapGAppsHook ];
   NIX_CFLAGS_COMPILE = "-I${glib.dev}/include/gio-unix-2.0";
   buildInputs = [ boost gtk3 ragel lua ];