The automatic detection uses the /proc interface proposed by the Nvidia
kernel module.
Having autodetection inside nix, I also changed the way the driver is
fetched, so everything is done inside nix. So no more `nvidiaInstall`
wrapper script.
I updated the `README` accordingly.
Hash of nvidia driver is not known because it depends on the driver version
(argument `nvidiaVersion`). However, since nix 2.0 and when using `useSandbox`,
`sha256 = null` is not allowed anymore.
The new wrapper script, `nvidiaInstall.py` uses `nix-prefetch-url` to compute
the hash and call `nix`.
This is a fix for #9
