fix(system): default physical installs to archlinux when os is omitted

roles/global_defaults/tasks/system.yml

@@ -29,7 +29,7 @@
       {{
         system.os
         if system.os is defined and (system.os | string | length) > 0
-        else ''
+        else ('archlinux' if (system_type_effective | lower) == 'physical' else '')
       }}
     system_os_version_effective: >-
       {{