Putting packages files in /var prevent makes new install start with an
already populated /var, which prevent taking advantage of the systemd
provided tmpfiles.d/var.conf, which notably has this line:
q /var 0755 - - -
-> on supported filesystems (currently that means btrfs), this creates
/var as a subvolume, which has the benefit of excluding it from /
snapshots.
On non-systemd system, the `meson install` behavior stays the same,
/var/lib/pacman and /var/cache/pacman/pkg are included in DESTDIR.
Signed-off-by: Max Gautier <mg@max.gautier.name>
