mkosi: (Archlinux) increase the size of root partition.

Currently "mkosi --default .mkosi/mkosi.arch" fails to build the image due to pacman error:

"error: Partition /var/tmp/mkosi-xxx/root too full: yyy blocks needed, zzz blocks free"
"error: not enough free disk space"
"error: failed to commit transaction (not enough free disk space)"

So increase the size to 3G.