fix: Remove bootstrap python setup from check mode

Since we can't look into the variable `pythoncheck.rc`, as it doesn't
exist, we skip the `when` check for checkmode -- its installation cannot
run under any circumstance in the mode anyways.

site.yaml

@@ -15,7 +15,11 @@
       register: pythoncheck
     - name: install debian python
       ansible.builtin.raw: apt-get update && apt-get install python3 -y
-      when: pythoncheck.rc == 127
+      when: not ansible_check_mode and pythoncheck.rc == 127
+    - name: pretend installing debian python for check mode
+      ansible.builtin.debug:
+        msg: Pretending to install python...
+      when: ansible_check_mode
 
 - name: Prepare incus server host
   hosts: host_system