nvidia: fix $ substitution in udev rules

This fixes a harmless error from systemd-udevd that looks like:

  Dec 23 15:35:23 dellbook systemd-udevd[696]:
  /nix/store/iixya3ni5whybpq9zz1h7f4pyw7nhd19-udev-rules/99-local.rules:25
  Invalid value "..." for RUN (char 101: invalid substitution type),
  ignoring, but please fix it.

Using $$ fixes it using the escaping documented at https://www.freedesktop.org/software/systemd/man/udev.html.

nixos/modules/hardware/video/nvidia.nix

@@ -197,10 +197,10 @@ in
     # Create /dev/nvidia-uvm when the nvidia-uvm module is loaded.
     services.udev.extraRules =
       ''
-        KERNEL=="nvidia", RUN+="${pkgs.runtimeShell} -c 'mknod -m 666 /dev/nvidiactl c $(grep nvidia-frontend /proc/devices | cut -d \  -f 1) 255'"
-        KERNEL=="nvidia_modeset", RUN+="${pkgs.runtimeShell} -c 'mknod -m 666 /dev/nvidia-modeset c $(grep nvidia-frontend /proc/devices | cut -d \  -f 1) 254'"
-        KERNEL=="card*", SUBSYSTEM=="drm", DRIVERS=="nvidia", RUN+="${pkgs.runtimeShell} -c 'mknod -m 666 /dev/nvidia%n c $(grep nvidia-frontend /proc/devices | cut -d \  -f 1) %n'"
-        KERNEL=="nvidia_uvm", RUN+="${pkgs.runtimeShell} -c 'mknod -m 666 /dev/nvidia-uvm c $(grep nvidia-uvm /proc/devices | cut -d \  -f 1) 0'"
+        KERNEL=="nvidia", RUN+="${pkgs.runtimeShell} -c 'mknod -m 666 /dev/nvidiactl c $$(grep nvidia-frontend /proc/devices | cut -d \  -f 1) 255'"
+        KERNEL=="nvidia_modeset", RUN+="${pkgs.runtimeShell} -c 'mknod -m 666 /dev/nvidia-modeset c $$(grep nvidia-frontend /proc/devices | cut -d \  -f 1) 254'"
+        KERNEL=="card*", SUBSYSTEM=="drm", DRIVERS=="nvidia", RUN+="${pkgs.runtimeShell} -c 'mknod -m 666 /dev/nvidia%n c $$(grep nvidia-frontend /proc/devices | cut -d \  -f 1) %n'"
+        KERNEL=="nvidia_uvm", RUN+="${pkgs.runtimeShell} -c 'mknod -m 666 /dev/nvidia-uvm c $$(grep nvidia-uvm /proc/devices | cut -d \  -f 1) 0'"
       '';
 
     boot.blacklistedKernelModules = [ "nouveau" "nvidiafb" ];