refactor: reuse nixgl.auto when nVidia is forced

Instead of duplicating its definition, because that leaves the
possibility for bugs when these duplicates aren't in sync.

Author: muggenhor

nixGL.nix

@@ -249,10 +249,5 @@ let
         nixGLCommon nixGLIntel;
     } // autoNvidia;
   };
-in top // (if nvidiaVersion != null then
-  top.nvidiaPackages {
-    version = nvidiaVersion;
-    sha256 = nvidiaHash;
-  }
-else
-  { })
+in top // lib.optionalAttrs (nvidiaVersion != null)
+(builtins.removeAttrs top.auto [ "nixGLDefault" ])