nix-cache: switch DNS to nix-cache02

- Move nix-cache CNAME from nix-cache01 to nix-cache02
- Remove actions1 CNAME (service removed)
- Update proxy.nix to serve canonical domain on nix-cache02
- Promote nix-cache02 to prod tier with build-host role

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

hosts/nix-cache02/configuration.nix

@@ -13,11 +13,13 @@
     ../../common/vm
   ];
 
-  # Host metadata (adjust as needed)
   homelab.host = {
-    tier = "test";  # Start in test tier, move to prod after validation
+    tier = "prod";
+    role = "build-host";
   };
 
+  homelab.dns.cnames = [ "nix-cache" ];
+
   # Enable Vault integration
   vault.enable = true;