fix binary cache for nixos

[goertzenator]

The instructions for setting up the binary cache on NixOS produce the following error:

[root@r9:~]# nixos-rebuild switch  
error: The option `nix.settings' does not exist. Definition values:  
- In `/etc/nixos/configuration.nix':  
   {  
     substituters = [  
       "https://hydra.iohk.io"  
     ];  
     trusted-public-keys = [  
   ...  
(use '--show-trace' to show detailed location information) 

The PR fixes this. I am using this on my NixOS system and it seems to work fine.

[hamishmack]

It looks like we merged #1410 too soon. I presume you must have a slightly older NixOS. Can you please update this PR to include both old and new settings? It would be good if we could find out when nix.settings.trusted-public-keys support was added to NixOS, but if not just calling them "Newer NixOS" and "Older NixOS" would work.

[goertzenator]

Apologies; I will investigate.

[goertzenator]

Fixed.

So, this change arrived after nixos-21.11. Although 21.11 is the current stable release of NixOS, the next release is right around the corner and will make this PR obsolete. To keep things simpler and clearer going forward, I suggest not merging this PR.

[michaelpj]

I don't see any harm in having a correct addendum like this, I think let's just include it. Thanks!