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>

services/nix-cache/proxy.nix

@@ -1,10 +1,10 @@
 { pkgs, config, ... }:
 let
-  # nix-cache01 serves the canonical nix-cache.home.2rjus.net
-  # nix-cache02 serves nix-cache02.home.2rjus.net (for testing before DNS cutover)
+  # nix-cache02 serves the canonical nix-cache.home.2rjus.net
+  # nix-cache01 serves nix-cache01.home.2rjus.net (deprecated, pending decommission)
   hostname = config.networking.hostName;
   domain =
-    if hostname == "nix-cache01" then
+    if hostname == "nix-cache02" then
       "nix-cache.home.2rjus.net"
     else
       "${hostname}.home.2rjus.net";