you can just run the command in the terminal adding the package name for installation. That works fine and does not require root rights on the user.
However, interesting question is how to install AUR packages via pacmanxg which runs under su / sudo privileges and uses yaourt for AUR and consequently fails on the installation with the above message. I suppose, that’s where your original question comes from. I’m currently not on my Antergos install so cannot check myself, but maybe one can start pacmanxg with only user rights? Would only by a work around anyway, since one would need to know before start of pacmanxg from where an installation / update will be performed. Maybe there is a config entry somewhere …
PS: just booted into Antergos: pacmanxg can be started with user right (there is a corresponding button on the main window when you start it). Doing that you can install packages from AUR but not from the regular Antergos repos anymore as I suspected earlier.
Dell Latitude E6230 with Intel i7, 8GB RAM, 500 GB SSD
dual boot: Antergos and Debian Stretch both 64bit and running gnome-shell
surviving in a Windows world with VMware WS and Codeweavers Crossover