Add an actual equality rule to nfs-server-down alert

alerts/nfs-down.rules

 ## gitlab.com
 ALERT NFSServerDown
-  IF up{fqdn=~"nfs-file[0-9]+.stor.gitlab.com"}
+  IF up{fqdn=~"nfs-file[0-9]+.stor.gitlab.com"} == 0
   FOR 2m
   LABELS {severity="critical", pager="pagerduty"}
   ANNOTATIONS {