Update to a newer Haskell.nix and materialize the Nix expressions for PaymentServer

This reduces the amount of memory required to install PaymentServer by ~500MB.

It's called "side effects".
Look it up.

Remove the Stack-based steps from CI, leaving only the Nix-based steps

Only use Nix.  I can figure out how to cache the Nix stuff on CircleCI.  The
stack stuff is hard and not obviously worth the effort.

CI configuration requires it

this is absolutely the wrong way to do it