Enable nix cache

Reviewed-by: Aleix Boné <abonerib@bsc.es>
Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>

m/tent/configuration.nix

@@ -8,6 +8,7 @@
     ../module/ssh-hut-extern.nix
     ./monitoring.nix
     ./nginx.nix
+    ./nix-serve.nix
   ];
 
   # Select the this using the ID to avoid mismatches

m/tent/nix-serve.nix

@@ -0,0 +1,16 @@
+{ config, ... }:
+
+{
+  age.secrets.nixServe.file = ../../secrets/nix-serve.age;
+
+  services.nix-serve = {
+    enable = true;
+    # Only listen locally, as we serve it via ssh
+    bindAddress = "127.0.0.1";
+    port = 5000;
+
+    secretKeyFile = config.age.secrets.nixServe.path;
+    # Public key:
+    # jungle.bsc.es:pEc7MlAT0HEwLQYPtpkPLwRsGf80ZI26aj29zMw/HH0=
+  };
+}