[Resolve #1520] Fix test for cmd hook shell option

[iainelder]

The previous version of this test meant to use bash, but really used sh.

Improves first PR #1523.

PR Checklist

• [ ] Wrote a good commit message & description [see guide below].
• [ ] Commit message starts with [Resolve #issue-number].
• [ ] Added/Updated unit tests.
• [ ] Added/Updated integration tests (if applicable).
• [ ] All unit tests (poetry run tox) are passing.
• [ ] Used the same coding conventions as the rest of the project.
• [ ] The new code passes pre-commit validations (poetry run pre-commit run --all-files).
• [ ] The PR relates to only one subject with a clear title. and description in grammatically correct, complete sentences.

Approver/Reviewer Checklist

• [ ] Before merge squash related commits.

Other Information

Guide to writing a good commit

[iainelder]

@dboitnot , does this still work on Nix?