Show diff on failure in cabal-gild check action

[palas]

Changelog

- description: |
    Added diff to output of cardano-gild check action
  type:
  - test           # fixes/modifies tests

Context

It is nice to know what is wrong specifically with the formatting when CI fails, so this PR modifies the cardano-gild action to show a diff when it fails.

I am also using the opportunity to fix a wrong variable name.

Related PR on cardano-api: https://github.com/IntersectMBO/cardano-api/pull/605

How to trust this PR

This is what happens when format check fails: Failed run

Checklist

• [x] Commit sequence broadly makes sense and commits have useful messages
• [x] New tests are added if needed and existing tests are updated. See Running tests for more details
• [x] Self-reviewed the diff