nvidia-container-toolkit: add meta.mainProgram

Signed-off-by: Connor Baker <[e-mail]>

pkgs/by-name/nv/nvidia-container-toolkit/package.nix

···
   meta = {
     homepage = "https://gitlab.com/nvidia/container-toolkit/container-toolkit";
     description = "NVIDIA Container Toolkit";
+    mainProgram = "nvidia-ctk";
     license = lib.licenses.asl20;
     platforms = lib.platforms.linux;
     maintainers = with lib.maintainers; [