Use extra- for substituters and trusted-public-keys

From the nix manual:

    A configuration setting usually overrides any previous value.
    However, for  settings  that  take  a  list  of items, you can
    prefix the name of the setting by extra- to append to the previous
    value.

m/module/hut-substituter.nix

@@ -4,7 +4,7 @@
     # Don't add hut as a cache to itself
     assert config.networking.hostName != "hut";
     {
-      substituters = [ "http://hut/cache" ];
-      trusted-public-keys = [ "jungle.bsc.es:pEc7MlAT0HEwLQYPtpkPLwRsGf80ZI26aj29zMw/HH0=" ];
+      extra-substituters = [ "http://hut/cache" ];
+      extra-trusted-public-keys = [ "jungle.bsc.es:pEc7MlAT0HEwLQYPtpkPLwRsGf80ZI26aj29zMw/HH0=" ];
     };
 }