Allow the nginx user to access SSL certificates

Fixes this error:

> Failed assertions:
> - Certificate oliverdavies.uk (group=acme) must be readable by
service(s) nginx.service (user=nginx groups=nginx),
nginx-config-reload.service (user=root groups=)

hosts/nixedo/modules/nginx/default.nix

@@ -47,5 +47,7 @@ in
     };
   };
 
+  users.users.${config.services.nginx.user}.extraGroups = [ "acme" ];
+
   imports = [ ./www.oliverdavies.uk.nix ];
 }