server7: disable radvd default route

Radvd is not enabled, this is for purposes of documentation.

hosts/server7/yggdrasil-prefix.nix

@@ -15,6 +15,7 @@ in {
       interface enp2s0f1
       {
       	AdvSendAdvert on;
+      	AdvRouteLifetime 0;
       	prefix ${yggaddr.prefix}:/64 {
       		AdvOnLink on;
       		AdvAutonomous on;