Fix failing nix tests

[Hritik14]

Nix tests are failing. Probably some kind of dependency issue.
See more: https://github.com/nexB/vulnerablecode/runs/5094793358?check_suite_focus=true

[Hritik14]

@rolfschr, As you're our nix savior, can you please look into it ?

[rolfschr]

Hi @Hritik14, I can have a look into this. I see that the nix-related documentation has been removed from the readme. I guess that's ok. Still, could you please dig it up again an place it into /etc/nix? Thanks.

[Hritik14]

Thanks @rolfschr for responding. The nix docs have just been moved to RTD at https://vulnerablecode.readthedocs.io/en/latest/installation.html#using-nix as I couldn't do a quick and neat tl;dr to place on README.
We'll be glad to accept a tl;dr PR on readme if you'd like to.

[rolfschr]

@Hritik14 See #635.

[Hritik14]

@rolfschr Thank you so much! We have the green ticks now.