nixpkgs_exposed is going away, and the haskell overlay is independent
from that.
See also b/108, cl/2910
Change-Id: I3aea6dfc427a914f3f88146fd0b45d60dfd45a1a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2918
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>