CI: bump dependencies

Description

Updates nixpkgs and haskell.nix to newer versions. It's needed for #642 (closed), because we can't use danger package via nixpkgs until we update it.

There are problems with stack2cabal requiring old dependencies again, last time we had these problems we made a PR to update stack2cabal: https://github.com/hasufell/stack2cabal/pull/34. And it had no updates since then. For now there is a workaround in ci.nix to use older packages.

And hlint in CI is currently failing, because hlint was updated and it has new hints now. I need someone's help to fix them.

Related issue(s)

#642 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.