playbooks/mirrors: make archweb inclusion conditional

We do not really want to make all mirrors run mirrorcheck, so make it
conditional based on whether archweb_mirrorcheck_locations is defined.

playbooks/mirrors.yml

@@ -11,7 +11,7 @@
     - { role: nginx }
     - { role: syncrepo, tags: ['nginx'] }
     - { role: syncdebug, when: mirror_debug_packages is not defined or mirror_debug_packages }
-    - { role: archweb, archweb_site: false, archweb_services: false, archweb_mirrorcheck: true }
+    - { role: archweb, when: archweb_mirrorcheck_locations is defined, archweb_site: false, archweb_services: false, archweb_mirrorcheck: true }
     - { role: prometheus_exporters }
     - { role: promtail }
     - { role: fail2ban }