Add module to use hut as a binary substituter

m/module/hut-substituter.nix

@@ -0,0 +1,9 @@
+{ config, ... }:
+  # Don't add hut as a cache to itself
+  assert config.networking.hostName  != "hut";
+{
+  nix.settings = {
+    substituters = [ "https://jungle.bsc.es/cache" ];
+    trusted-public-keys = [ "jungle.bsc.es:pEc7MlAT0HEwLQYPtpkPLwRsGf80ZI26aj29zMw/HH0=" ];
+  };
+}