fix: correct regex to capture negative objective values when using CBC

[qheuristics]

Currently returns np.nan if objective function value solved by CBC is negative

Closes # (if applicable).

Changes proposed in this Pull Request

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.