monitoring: fix nix-cache_caddy scrape target TLS error

Move nix-cache_caddy back to a manual config in prometheus.nix using the
service CNAME (nix-cache.home.2rjus.net) instead of the hostname. The
auto-generated target used nix-cache01.home.2rjus.net which doesn't
match the TLS certificate SAN.

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

services/nix-cache/default.nix

@@ -7,9 +7,4 @@
     ./nix.nix
   ];
 
-  homelab.monitoring.scrapeTargets = [{
-    job_name = "nix-cache_caddy";
-    port = 443;
-    scheme = "https";
-  }];
 }