fix: point litellm provider source at the artifactapi registry in root.hcl

The module terraform.tf files use the artifactapi terraform-unkin registry
as the provider source, but the generated root backend.tf still referenced
git.unkin.net, which is not a valid OpenTofu provider registry host and
failed the plan pipeline. Align root.hcl with the modules.

environments/root.hcl

@@ -36,7 +36,7 @@ terraform {
       version = "2.23.0"
     }
     litellm = {
-      source  = "git.unkin.net/unkin/litellmvaultsecret"
+      source  = "artifactapi.k8s.syd1.au.unkin.net/terraform-unkin/litellmvaultsecret"
       version = "0.1.0"
     }
   }