ci/lava: Handle proxy download failures
LAVA doesn't consider failure to download a kernel/initramfs as an infrastructure error, rather just a user error for supplying a broken URL. We know our URLs aren't broken (because we're perfect), so assume that failures in download validation are network issues and retry when we hit them.
LAVA itself has been fixed to retry internally, so we'll get that when upgrade in a couple of weeks, but gloss over it for now.