Right, makes sense. Your options are
- Connect via a wired connection temporarily just to get this working.
- Boot back into the installer, connect to the wifi there, mount your filesystems like when you originally installed, and rerun nixos-install. It will only fetch the newly necessary stuff and build a new generation of your installed system, and when you then boot back into the installed system wpa_supplicant.service should be running.
Hope this helps! 