Update nixpkgs snapshot
Update nixpkgs snapshot

This upgrades GHC from 8.8.2 to 8.8.4. This is necessary to stay compatible with the
master branch of nixpkgs. Icepeak was marked as broken there, since it
couldn't be built any more with the GHC and libraries available there.

channable/icepeak,channable/icepeak,channable/icepeak