fix: tell Ansible about the idempotency of this task

This shell command doesn't change state on the system, therefore
it is safe to set `changed_when` to `false`, and we can also
set `check_mode` to `false` here to allow execution when run in
check mode.

ansible/maintenance/maint-diskspace.yaml

@@ -9,6 +9,8 @@
           set -euo pipefail
           df -Ph / | awk 'NR==2 {print $5}'
         executable: /bin/bash
+      changed_when: false
+      check_mode: false
       register: disk_usage
 
     # - name: Send discord message when disk space is over 80%