support/download: convert scp to use the wrapper

This drastically simplifies the scp helper, as it no longer has to deal
with atomically saving the downloaded archive.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
(Tested by setting a primary site to 'scp://localhost:/tmp

' and
running 'make vim-source')
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>