Ignore the filesystem package for Arch Linux.

The filesystem package cannot typically be updated in a docker image
because of the read-only `/etc/resolv.conf` and `/etc/hosts`.  I don't
see a reason not to remove it from the standard sysupgrade (pacman -Syu)
command.

Fixes #7075

Docker-DCO-1.1-Signed-off-by: Spencer Rinehart <anubis@overthemonkey.com> (github: nubs)