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-cache01/configuration.nix

@@ -11,8 +11,6 @@
     ../../common/vm
   ];
 
-  homelab.dns.cnames = [ "nix-cache" "actions1" ];
-
   homelab.host.role = "build-host";
 
   fileSystems."/nix" = {