fix: Set __is_system_running in check mode

Cause: `systemctl is-system-running` was not called in `--check` mode.

Consequence: The "Determine if system is booted with systemd" step
failed in check mode as the `__is_system_running` variable was not
populated.

Fix: Force calling `systemctl is-system-running` in check mode. It does
not modify the system and the outcome is very influential for what the
role does.

Author: martinpitt

tasks/set_vars.yml

@@ -25,6 +25,7 @@
       command: systemctl is-system-running
       register: __is_system_running
       changed_when: false
+      check_mode: false
       failed_when: false
 
     - name: Require installed systemd