[#26] Run bats tests in pipeline

[dcastro]

Description

Problem: we have golden tests for the coffer CLI written in bats, but we're not running them on the pipeline yet.

Solution: Add a step to the buildkite script to run the golden tests via nix.

Related issue(s)

Resolves part of #26

:white_check_mark: Checklist for your Pull Request

Related changes (conditional)

• Tests

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

• Documentation

  • [x] I checked whether I should update the docs and did so if necessary:
  • README
  • Haddock

• Public contracts

  • [ ] Any modifications of public contracts comply with the Evolution of Public Contracts policy.
  • [ ] I added an entry to the changelog if my changes are visible to the users and
  • [ ] provided a migration guide for breaking changes if possible

Stylistic guide (mandatory)

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

[dcastro]

Logs of a successful run: https://buildkite.com/serokell/coffer/builds/85#9950329d-39dd-4764-b46e-80d5d7fc5526

[MagicRB]

LFTM (Looks Functional To Me) - I'd have done it a bit differently (only using flakes) but that'll work too