CI: pin hackage and stackage indexes independently of haskell.nix

Description

Problem: we use default hackage and stackage pins provided by haskell.nix. It means that when a developer wants to add a new dependency, they have to bump haskell.nix, which means that CI might need to rebuild GHC and all haskell dependencies, which takes a long time

Solution: use separate pins for hackage and stackage indexes, so that they can be updated independently

Related issue(s)

None

✅ 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.

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• My code complies with the style guide.