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.
(From OE-Core rev: a3284958a2cc6c90a5fac26976bddc23f821c972)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>