/etc/ifplugd/action.d/action_wpa is in wpasupplicant 2.4-0ubuntu6.
This file is owned by root:root, with mode 0o777.
It is a symlink to /etc/ifplugd/action.d/../../wpa_supplicant/action_wpa.sh
/etc/ifplugd/action.d/../../wpa_supplicant/action_wpa.sh