Use Nix to find VPN network IP

Thanks to @jcalderone for the suggestion!

nixos/modules/monitoring/server/grafana.nix

@@ -187,7 +187,7 @@ in {
           # Only allow our monitoringvpn subnet
           # And localhost since we're the monitoring server currently
           extraConfig = ''
-            allow 172.23.23.0/24;
+            allow ${config.grid.monitoringvpnIPv4}/24;
             allow 127.0.0.1;
             allow ::1;
             deny all;