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.

Reviewed-by: Rodrigo Arias Mallo <rodrigo.arias@bsc.es>

m/raccoon/configuration.nix

@@ -29,8 +29,8 @@
   };
 
   nix.settings = {
-    substituters = [ "https://jungle.bsc.es/cache" ];
-    trusted-public-keys = [ "jungle.bsc.es:pEc7MlAT0HEwLQYPtpkPLwRsGf80ZI26aj29zMw/HH0=" ];
+    extra-substituters = [ "https://jungle.bsc.es/cache" ];
+    extra-trusted-public-keys = [ "jungle.bsc.es:pEc7MlAT0HEwLQYPtpkPLwRsGf80ZI26aj29zMw/HH0=" ];
   };
 
   # Enable performance governor