Get timestamp even in check mode

We want to set the _timestamp fact, even when we're running
"ansible-playbook --check". Ansible skips any tasks using the shell
and command modules in check mode by default, so we have to force
running the "date" command by setting the (confusingly named)
"check_mode: no" option on that task.

roles/taiga/tasks/main.yml

@@ -5,6 +5,7 @@
   register: date
   when: _timestamp is not defined
   changed_when: false
+  check_mode: no
   tags:
     - always