Unquote double quoted shell values

[jfrost-mo]

This handles the double quoting that arises from CSET_ADDOPTS arguments never actually being parsed by a shell.

Contribution checklist

Aim to have all relevant checks ticked off before merging. See the developer's guide for more detail.

• [ ] Documentation has been updated to reflect change.
• [ ] New code has tests, and affected old tests have been updated.
• [ ] All tests and CI checks pass.
• [ ] Ensured the pull request title is descriptive.
• [ ] Conda lock files have been updated if dependencies have changed.
• [ ] Attributed any Generative AI, such as GitHub Copilot, used in this PR.
• [ ] Marked the PR as ready to review.

[github-actions[bot]]

[jfrost-mo]

Ack. More investigation is needed then...

[jfrost-mo]

There are still some issues with this method. The wrong types of quotes are being expanded, for one.

[jfrost-mo]

Should be a more complete fix now, and is hopefully also more readable?

[jfrost-mo]

Rebased then merged.