feat: refactor domain whitelist in rspamd.nix

This update introduces a more efficient way for managing whitelisted domains in rspamd.nix. Instead of repeating the list of domains across multiple configurations, the domains are now defined only once in a dedicated variable. This improves the maintainability and readability of the code.

systems/x86_64-linux/mx/rspamd.nix

@@ -1,4 +1,14 @@
 { pkgs, lib, ... }:
+let
+  domains = ''
+    epicgames.com
+    dmail.ai
+    twitter.com
+    x.com
+    gmx.de
+    chess.com
+  '';
+in
 {
   services.rspamd.workers.controller.bindSockets = [{
     socket = "/run/rspamd/worker-controller.sock";
@@ -17,34 +27,10 @@
         }
       }
     '';
-    "maps.d/spf_whitelist.inc.local".text = ''
-      epicgames.com
-      dmail.ai
-      twitter.com
-      x.com
-      gmx.de
-    '';
-    "maps.d/spf_dkim_whitelist.inc.local".text = ''
-      epicgames.com
-      dmail.ai
-      twitter.com
-      x.com
-      gmx.de
-    '';
-    "maps.d/dmarc_whitelist.inc.local".text = ''
-      epicgames.com
-      dmail.ai
-      twitter.com
-      x.com
-      gmx.de
-    '';
-    "greylist-whitelist-domains.inc".text = ''
-      dmail.ai
-      epicgames.com
-      twitter.com
-      x.com
-      gmx.de
-    '';
+    "maps.d/spf_whitelist.inc.local".text = domains;
+    "maps.d/spf_dkim_whitelist.inc.local".text = domains;
+    "maps.d/dmarc_whitelist.inc.local".text = domains;
+    "greylist-whitelist-domains.inc".text = domains;
   };
   services.rspamd.extraConfig = ''
     actions {