Fix mirrorlist: flat URL without $repo/os/$arch

Repo packages are at root of GitHub Pages, not in
$repo/os/$arch subdirectory structure. Mirrorlist
must match the flat layout.

packages/antergos-next-mirrorlist/antergos-next-mirrorlist

@@ -2,4 +2,4 @@
 ## Generated by antergos-pkgs CI
 ## Use pacman-mirrors or rankmirrors to select fastest mirror
 
-Server = https://antergos-next.github.io/antergos-pkgs/$repo/os/$arch
+Server = https://antergos-next.github.io/antergos-pkgs