Fix Flatcar bug #11268 missing default value for...

Fix Flatcar bug #11268 missing default value for ansible_interpreter_python_fallback variable (#11270)

roles/bootstrap-os/tasks/flatcar.yml

@@ -23,7 +23,7 @@
 
 - name: Make interpreter discovery works on Flatcar
   set_fact:
-    ansible_interpreter_python_fallback: "{{ ansible_interpreter_python_fallback + [ '/opt/bin/python' ] }}"
+    ansible_interpreter_python_fallback: "{{ (ansible_interpreter_python_fallback | default([])) + [ '/opt/bin/python' ] }}"
 
 - name: Disable auto-upgrade
   systemd_service: