Add module to use hut as a binary substituter

m/module/hut-substituter.nix

@@ -0,0 +1,10 @@
+{ config, ... }:
+{
+  nix.settings =
+    # Don't add hut as a cache to itself
+    assert config.networking.hostName != "hut";
+    {
+      substituters = [ "https://jungle.bsc.es/cache" ];
+      trusted-public-keys = [ "jungle.bsc.es:pEc7MlAT0HEwLQYPtpkPLwRsGf80ZI26aj29zMw/HH0=" ];
+    };
+}