sysutils/pacman: fix packaging archlinux flavour on official builders

Apparently ${PREFIX} is strictly enforced and prepended on entries
in ${PLIST} on the official builders so trying to list anything
outside of ${PREFIX} like /compat ends up as ${PREFIX}/compat, so
move these entries to ${PLIST_DIRS}. The ${PLIST} listing approach
results in errors like:

> Building archlinux-pacman-7.1.0

pkg-static: Unable to access file /wrkdirs/usr/ports/sysutils/pacman/work-archlinux/stage/usr/local/compat/archlinux/var/cache/pacman/pkg/:No such file or directory

Reported by: fallout