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.