Hack to prevent <interface>-cfg.service from breaking the default gateway

Restarting <interface>-cfg.service may cause the interface's IP
addresses to be flushed.  If the default gateway goes through that
interface, then the default gateway is deleted.  So we need to
restart network-setup.target.

modules/tasks/network-interfaces.nix

@@ -347,6 +347,9 @@ in
                     echo "configuring interface..."
                     ip -4 addr flush dev "${i.name}"
                     ip -4 addr add "${i.ipAddress}/${mask}" dev "${i.name}"
+                    # Ensure that the default gateway remains set.
+                    # (Flushing this interface may have removed it.)
+                    ${config.system.build.systemd}/bin/systemctl try-restart --no-block network-setup.service
                   else
                     echo "skipping configuring interface"
                   fi