pkg-infra: also set PKGDIR for the download step

This will be needed to get the hash file, to check the
downloaded files.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>