[Chore] Don't use old 'nix run' command

[rvem]

Description

Problem: 'nix run' interface has changed with flake addition. However, CI pipeline in this repo still uses the old 'nix run' for which we keep a workaround on our build servers.

Solution: Use 'nix shell' instead.

Related issue(s)

Fixed #

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Related changes (conditional)

• Tests

  • [ ] 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:
  • README
  • Haddock

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• [ ] My commits comply with the policy used in Serokell.
• [ ] My code complies with the style guide.