refactor(nixos): derive AdGuard proxy rewrites

configs/nixos/guests/adguard.nix

@@ -1,6 +1,7 @@
 {
   config,
   lib,
+  registry,
   ...
 }:
 let
@@ -34,6 +35,7 @@ in
         modules.services.adguardhome = {
           enable = true;
           inherit (cfg) adminPasswordHash;
+          reverseProxyAddress = registry.address.${config.networking.hostName}."reverse-proxy";
         };
 
         services.resolved.settings.Resolve.DNSStubListener = false;