I had the same issue, although there didn't seem to be any way to set the fork in Xcode as like you say it seems to stubornly set it to the original repo
I did have success editing the project file in a text editor: {project}.xcodeproj/project.pbxproj and update the repositoryURL there
That prompted Xcode to fetch the dependencies again, this time of the fork
Topic:
Developer Tools & Services
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Xcode
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