Use 'downloads.haskell.org' instead of GitHub assets

GitHub serves files from a single geographical location, severely reducing
download speeds for non-US users. Even worse, some (European) ISPs seem
to have peering issues limiting download speeds to sub-1 Mbps at times.

This commit changes 'stack-setup-2.yaml' to use
'downloads.haskell.org' instead of GitHub Assets. At the time of
writing, the former uses a proper CDN (Fastly) to deliver content which
should result in consistent speeds around the globe.

Discussion on:

  commercialhaskell/stack#2240