actionlint: provision it in Nix shell, and use the same in CI

[smelc]

Changelog

- description: |
    actionlint: provision it in Nix shell, and use the same in CI
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

• Implementation of CI: add shellcheck and actionlint cardano-node#5804 (comment) in CLI
• This is a nice change because it makes actionlint available to Nix users automatically and also ensures that the version used by the CI is the same one as in Nix shells.

How to trust this PR

• Load the Nix shell locally and observe you have actionlint in your PATH.
• Look at the execution of the actionlint pipeline on this PR and witness it works like before: https://github.com/IntersectMBO/cardano-cli/actions/runs/9095975027/job/25000363605?pr=767

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• Self-reviewed the diff