fix(global_defaults): read dict2items key and key enrichment on os

roles/global_defaults/tasks/system.yml

@@ -35,10 +35,12 @@
         }, recursive=True)
       }}
 
+# Keyed on os, not hostname: inventories set hostname themselves, and doing so
+# would otherwise skip enrichment and leave os undefined for validation.
 - name: Check if pre-computed system_cfg needs enrichment
   when: system_cfg is defined
   ansible.builtin.set_fact:
-    _bootstrap_needs_enrichment: "{{ hostname is not defined }}"
+    _bootstrap_needs_enrichment: "{{ os is not defined }}"
 
 - name: Merge pre-computed system_cfg with bootstrap system_defaults
   when: