oeqa/sdk/case: Ensure DL_DIR is populated with artefacts if used

Where we're using DL_DIR in sdk archive to try and cache testing artefacts,
copy into the cache so that it gets populated and this doesn't have to be done
manually. Currently we're making a lot of repeat requests to github as this
wasn't being populated.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit a3284958)
Signed-off-by: Steve Sakoman <steve@sakoman.com>