fix(molecule): serialize guest fact gathering

- Avoid concurrent SSH startup bursts against the disposable guests.
- Ensure every guest gathers interface facts before network setup.
- Prevent unreachable hosts from falling back to the default eth0 interface.

molecule/ipv6/overrides.yml

@@ -1,6 +1,7 @@
 ---
 - name: Apply overrides
   hosts: all
+  serial: 1
   tasks:
     - name: Override host variables (1/2)
       ansible.builtin.set_fact: