I’m afraid you’re going to have to repeat the same thing with the ‘core’ snap, i.e. download it via some other means, rename it as
snap download expects, and go from there.
Alternatively, if you’re feeling brave, I wrote the fix for this and
while it’s not even in master yet you could grab the snap binary built from that branch, and run download using that. Something like
wget -O /tmp/snap.bz2 https://people.canonical.com/~john/snap__amd64__PR7315_download_leave_partial.bz2 bunzip2 /tmp/snap.bz2 chmod +x /tmp/snap /tmp/snap download core