hosts: nixos: porthos: system: disable cache

We're never going to get a cache hit, since this is the host for that
cache.

hosts/nixos/porthos/default.nix

@@ -9,6 +9,7 @@
     ./networking.nix
     ./secrets
     ./services.nix
+    ./system.nix
     ./users.nix
   ];
 

hosts/nixos/porthos/system.nix

@@ -0,0 +1,12 @@
+# Core system configuration
+{ ... }:
+{
+  my.system = {
+    nix = {
+      cache = {
+        # This server is the one serving the cache, don't try to query it
+        selfHosted = false;
+      };
+    };
+  };
+}