libnvidia-container: set mainProgram

(cherry picked from commit 42ed2f841a08e8b35492a3ffd7b38e2863e5c957)

pkgs/applications/virtualization/libnvidia-container/default.nix

···
     description = "NVIDIA container runtime library";
     license = licenses.asl20;
     platforms = platforms.linux;
+    mainProgram = "nvidia-container-cli";
     maintainers = with maintainers; [ cpcloud ];
   };
 }