Make all pipelines pass actionlint and remove useless bits in flake.nix

[smelc]

Changelog

- description: |
    Make all pipelines file pass actionlint and remove useless bits in flake.nix
# 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
  # - improvement    # QoL changes e.g. refactoring
  # - 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

Follow-up of #740 and IntersectMBO/cardano-api#525

Checklist

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

[carbolymer]

do we need this?

[smelc]

We don't need it, but I think it's good practice to be able to trigger the pipelines manually. It comes handy when the pipeline starts failing and needs quick trial/error fixing.