Add Hydra binary cache to common config

lib/common/c3d2.nix

@@ -1,11 +1,18 @@
 { config, pkgs, ... }:
 
 {
-  imports = [./common.nix];
+  imports = [ ./common.nix ];
 
-  networking = {
-    domain = "hq.c3d2.de";
-    defaultGateway.address = "172.22.99.1";
+  users.motd = builtins.readFile ./motd;
+
+  networking.domain = "hq.c3d2.de";
+
+  nix = if config.services.nix-serve.enable then
+    { }
+  else {
+    binaryCaches = [ "https://nix-serve.hq.c3d2.de" ];
+    binaryCachePublicKeys =
+      [ "nix-serve.hq.c3d2.de:FEi9GyFkou1Ua8INaEKmuGaww9E5y3XwrNGNRfKYeLo=" ];
   };
 
 }

lib/users.nix

@@ -1,8 +1,6 @@
 { pkgs, ... }:
 
 {
-  users.motd = builtins.readFile ./motd;
-
   users.users.k-ot = {
     packages = with pkgs;
       [ screen tmux ];