reproducible Arch Linux: Add variation for /bin/sh

Install dash in the chroot, and symlink it as /bin/sh for the first
build. Also use `pacman --needed` to skip explicitly reinstalling
already-installed packages.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Holger Levsen <holger@layer-acht.org>