Let Sage recognize the location of new-installed nauty

[Chin-Hung Lin]

Hi, 

I am running the CoCalc docker on an Ubuntu machine.  The system inside the container did not have `nauty` installed, so `graphs.nauty_geng` did not work.

I then use `apt install nauty` to install the app.  

After installation, the app `nauty-geng` is available in the system.  

However, the `graphs.nauty_geng` function seems calling the app `geng` instead of `nauty-geng`.  

May I ask for some suggestions on how to set the configuration correctly so that Sage recognize the right location of `nauty`?

OS: Ubuntu 20.04

Sage: 9.2

nauty: 2.6r10, installed by apt as root of the system inside the container (but I have tested that a normal user is able to access `nauty-geng`; it seems not a permission issue, it's more like Sage cannot find the right place)

Source code of graphs.nauty_geng gives me

```

sp  = subprocess.Popen(nautyprefix+"geng {0}".format(options) ...

```

and I checked that `nautyprefix` is set as an empty string.