Use hut nix cache in owl1, owl2 and raccoon

For owl1 and owl2 directly connect to hut via LAN with HTTP, but for
raccoon pass via the proxy using jungle.bsc.es with HTTPS. There is no
risk of tampering as packages are signed.

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

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 = [ "http://hut/cache" ];
+      trusted-public-keys = [ "jungle.bsc.es:pEc7MlAT0HEwLQYPtpkPLwRsGf80ZI26aj29zMw/HH0=" ];
+    };
+}