nix-cache02: add homelab-deploy builder service

- Configure builder to build nixos-servers and nixos (gunter) repos
- Add builder NKey to Vault secrets
- Update NATS permissions for builder, test-deployer, and admin-deployer
- Grant nix-cache02 access to shared homelab-deploy secrets

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

hosts/nix-cache02/default.nix

@@ -1,5 +1,6 @@
 { ... }: {
   imports = [
     ./configuration.nix
+    ./builder.nix
   ];
 }