v-m/alerts: Ignore Restic alert for bw0

_bw0.pyrocufflink.blue_ has been decommissioned since some time, so it
doesn't get backed up any more.  We want to keep its previous backups
around, though, in case we ever need to restore something.  This
triggers the "no recent backups" alert, since the last snapshot is over
a week old.  Let's ignore that hostname when generating this alert.

victoria-metrics/alerts.yml

@@ -246,6 +246,7 @@ groups:
   - alert: Last Backup Age
     expr: >-
       time() - restic_backup_timestamp{
+        client_hostname!="bw0.pyrocufflink.blue",
         client_hostname!="luma.pyrocufflink.blue",
         client_hostname!="toad.pyrocufflink.blue",
       }> 604800