chore(nix): Bump dependencies.

Switch back to the haskell.nix-provided HLS.

Also, we can undo the workarounds needed for
input-output-hk/haskell.nix#1177 as that
issue has now been fixed upstream.

And one more thing: we need to disable the NixOS test framework for
the time being until I can port it to the next NixOS test system.

Author: dhess