Set the checkout actions fetch depth in build gate workflow to ensure there is a local version of the main branch available (#110)

# Pull Request

## 📖 Description

<!--- Provide some background and a description of your work. -->
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

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Provide some notes for reviewers to help them provide targeted feedback and testing.
-->
See: https://github.com/actions/checkout#checkout-v2

## ✅ Checklist

### General

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

.github/workflows/ci-validate.yml

@@ -18,6 +18,8 @@ jobs:
     
     steps:
     - uses: actions/checkout@v2
+      with:
+          fetch-depth: 0
 
     - name: Cache multiple paths
       uses: actions/cache@v2