fix: nvidia-docker + cgroups v2 disable unified hierarchy asdfasdfasdf

Took an entire day to find the linked comment [1] by @biggs, which says:

> Fix on NixOS (where cgroup v2 is also now default): add
> `systemd.enableUnifiedCgroupHierarchy = false;`
> and restart.

Indeed, after applying this commit and then running
`sudo systemctl restart docker`, any of the following commands works:

```bash
sudo docker run --gpus=all nvidia/cuda:10.0-runtime nvidia-smi
sudo docker run --runtime=nvidia nvidia/cuda:10.0-runtime nvidia-smi
sudo nvidia-docker run nvidia/cuda:10.0-runtime nvidia-smi
```

ARGH!!!1

Links:
[1] NVIDIA/nvidia-docker#1447 (comment)
[2] NixOS/nixpkgs#127146
[3] NixOS/nixpkgs#73800
[4] https://blog.zentria.company/posts/nixos-cgroupsv2/

P.S.
I use Colemak, but typing arstarstarst doesn't have the same ring to it.

configuration.nix

@@ -88,7 +88,12 @@
     '';
   };
 
-  virtualisation.docker.enable = true;
+  systemd.enableUnifiedCgroupHierarchy = false;
+
+  virtualisation.docker = {
+    enable = true;
+    enableNvidia = true;
+  };
 
   virtualisation.virtualbox.host = {
     enable = true;