Add explicit remote when using $GITHUB_BASE_REF

[bd-g]

Proposed changes

When submitting #29, I noticed the CI action succeeded but had errors. These errors didn't occur locally when running with act. After digging into it, I found this issue with the checkout GitHub action that seems to suggest we need to explicitly define the remote for $GITHUB_BASE_REF for it to work.

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Documentation update or tests (if none of the other choices apply)

Checklist

• [x] I have read the CONTRIBUTING doc
• [ ] I have tested my changes in my local self-hosted environment
  • As this action is specific to remote GH actions and I can't reproduce locally, I'm not sure how to test locally
• [x] I have added necessary documentation (if appropriate)