Another simple flake for Haskell development

I feel like that is more of an issue with flakes, rather than an issue with the default Haskell template:

You can see that there was a bunch of push-back from that, mostly from Haskellers.

I have a little more explanation here: "'allow-import-from-derivation' is disabled" error · Issue #34 · purenix-org/purenix · GitHub