fix: correct var for source build ref

[roleyfoley]

Description

Minor fix to the Url image source function to use the correct variable for the build ref source

Motivation and Context

Allows you to see where an artefact came from in a URL based image source

How Has This Been Tested?

Tested locally

Types of changes

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] Refactor (non-breaking change which improves the structure or operation of the implementation)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Followup Actions

• [x] None

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.
• [X] None of the above.