nixos/opengl: add LD_LIBRARY_PATH to systemVariables instead

so all users get this variable, thanks to work from #2692.

Author: vcunat

nixos/modules/hardware/opengl.nix

@@ -16,7 +16,7 @@ let
       [ p.mesa_drivers
         p.mesa_noglu # mainly for libGL
         (if cfg.s3tcSupport then p.libtxc_dxtn else p.libtxc_dxtn_s2tc)
-	p.udev
+        p.udev
       ];
   };
 
@@ -101,7 +101,7 @@ in
         ''}
       '';
 
-    environment.variables.LD_LIBRARY_PATH =
+    environment.systemVariables.LD_LIBRARY_PATH =
       [ "/run/opengl-driver/lib" "/run/opengl-driver-32/lib" ];
 
     # FIXME: move this into card-specific modules.