Fix using of absolute paths in prettify script

[carbolymer]

Changelog

- description: |
    Fix using of absolute paths in prettify script
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  # - 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
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[Jimbo4350]

What about skipping files containing CPP?

[carbolymer]

@Jimbo4350 it's done in prettify itself already, so there was no point in duplicating the logic. You can see it in the CI output: https://github.com/IntersectMBO/cardano-api/actions/runs/10528514101/job/29174180469?pr=621#step:5:23