Silently ban OpenVAS BSC scanner from apex

It is spamming our logs with refused connection lines:

apex% sudo journalctl -b0 | grep 'refused connection.*SRC=192.168.8.16' | wc -l
13945

Reviewed-by: Aleix Boné <abonerib@bsc.es>

m/apex/configuration.nix

@@ -65,6 +65,14 @@
       ProxyJump raccoon
   '';
 
+  networking.firewall = {
+    extraCommands = ''
+      # Blackhole BSC vulnerability scanner (OpenVAS) as it is spamming our
+      # logs. Insert as first position so we also protect SSH.
+      iptables -I nixos-fw 1 -p tcp -s 192.168.8.16 -j nixos-fw-refuse
+    '';
+  };
+
   # Use tent for cache
   nix.settings = {
     extra-substituters = [ "https://jungle.bsc.es/cache" ];