prometheus: Disable the OOM alert for build hosts

There is not much value in knowing when one of our build hosts has no
more memory left as all of them have plenty of swap available.
Additionally these rules trigger quite often even for short spikes.

Signed-off-by: Christian Heusel <christian@heusel.eu>

roles/prometheus/files/node.rules.yml

@@ -21,7 +21,7 @@ groups:
           description: "Swap is filling up (>80%)\n  VALUE = {{ $value }}\n  LABELS: {{ $labels }}"
 
       - alert: HostOutOfMemory
-        expr: node_memory_MemAvailable_bytes / node_memory_MemTotal_bytes * 100 < 10
+        expr: node_memory_MemAvailable_bytes{instance!~"build.archlinux.org",instance!~"repro3.pkgbuild.com",instance!~"repro2.pkgbuild.com"} / node_memory_MemTotal_bytes * 100 < 10
         for: 5m
         labels:
           severity: warning