tasks/upgrade-servers: tweak pacman upgrade logic

Avoid updating the cache in the same task w/ the upgrade as the former
causes the combined task to always return changed=True. For up-to-date
hosts, stop early instead of following through to the end and skipping
the final reboot task.

Before Ansible 5.4.0, combined cache update + package upgrade would not
always return changed=True but instead depended on whether the were any
packages to upgrade.

playbooks/tasks/include/upgrade-server.yml

@@ -8,10 +8,13 @@
 
 - name: upgrade all packages
   pacman:
-    update_cache: yes
     upgrade: yes
   register: pacman_upgrade
 
+- name: stop if no packages were upgraded
+  meta: end_host
+  when: pacman_upgrade is not changed
+
 - name: check for running builds
   block:
     - name: list build-related processes
@@ -59,4 +62,3 @@
 
 - name: reboot
   reboot:
-  when: pacman_upgrade is changed