Set the checkout actions fetch depth in build gate workflow to ensure there is a local version of the main branch available

[janechu]

Pull Request

📖 Description

The build gate must have a main branch available to check against when assessing if change files need to be present. This accomplishes this by setting the fetch depth to 0, which will during checkout, check all branches out for reference.

👩‍💻 Reviewer Notes

See: https://github.com/actions/checkout#checkout-v2

✅ Checklist

General

• [ ] I have added tests for my changes.
• [x] I have tested my changes.
• [ ] I have updated the project documentation to reflect my changes.