debian 8: change jessie-backport repo URL
debian 8: change jessie-backport repo URL

Since jessie-backports repo is no longer maintained, repo URL need to
change to archive.debian.org, also need to specify
Acquire::Check-Valid-Until=false.

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

scylladb/scylla-artifact-tests,scylladb/scylla-artifact-tests