Allow PR validate workflow to manually be dispatched

[janechu]

Pull Request

📖 Description

Allow the PR validate step to be manually dispatched. This will be helpful for testing against any branch prior to opening a PR.

✅ Checklist

General

• [ ] I have included a change request file using $ yarn change
• [ ] I have added tests for my changes.
• [ ] I have tested my changes.
• [ ] I have updated the project documentation to reflect my changes.
• [x] I have read the CONTRIBUTING documentation and followed the standards for this project.