Your mirrorlist is outdated. You must download the pacman package and untar it from /:
% cd /
% wget https://mirrors.dotsrc.org/artix-linux/repos/system/os/x86_64/pacman-5.0.2-9-x86_64.pkg.tar.xz -O /tmp/pacman-5.0.2-9-x86_64.pkg.tar.xz
% sudo tar Jxvf /tmp/pacman-5.0.2-9-x86_64.pkg.tar.xz
Now make this in your /etc/pacman.d/mirrorlist
# Artix mirrors
Server = http://mirror1.artixlinux.org/repos/$repo/os/$arch
Server = http://artix.wheaton.edu/repos/$repo/os/$arch
Server = http://mirror.strits.dk/artix-linux/repos/$repo/os/$arch
Server = https://mirrors.dotsrc.org/artix-linux/repos/$repo/os/$arch
Server = https://www.uex.dk/public/artix/$repo/os/$arch
And re-install pacman with pacman (so you can pacman while you pacman):
% sudo pacman -Sy pacman
% sudo pacman -Su