Fix removal of toScriptIndex export

[carbolymer]

Changelog

- description: |
    Fix removal of toScriptIndex export
# 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

Fixes: https://github.com/IntersectMBO/cardano-api/pull/436/files#diff-b9922f7294df3af9d6e30d914012d749b265d746ed35b9ea75ab69cddafeab87L147

This PR results in no change in visible API since 8.38 release, so it has a type maintenance.

Checklist

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