Enable automatic Nix GC in raccoon

The NVIDIA GTX 960 from 2016 has the Maxwell architecture, and NixOS
suggests using the proprietary driver for older than Turing:

> It is suggested to use the open source kernel modules on Turing or
> later GPUs (RTX series, GTX 16xx), and the closed source modules
> otherwise.

m/raccoon/configuration.nix

@@ -51,13 +51,11 @@
 
   # Configure Nvidia driver to use with CUDA
   hardware.nvidia.package = config.boot.kernelPackages.nvidiaPackages.production;
+  hardware.nvidia.open = false; # Maxwell is older than Turing architecture
   hardware.graphics.enable = true;
   nixpkgs.config.nvidia.acceptLicense = true;
   services.xserver.videoDrivers = [ "nvidia" ];
 
-  # Disable garbage collection for now
-  nix.gc.automatic = lib.mkForce false;
-
   services.openssh.settings.X11Forwarding = true;
 
   services.prometheus.exporters.node = {