Not sure if this was fixed by something external, but in case it helps anyone, it started working for me after going Xcode -> Settings -> Accounts -> Source Control Accounts -> GitHub and switching "Clone Using:" to SSH and uploading an SSH key
keenan-chiasson
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Builds working for me again as well! Looks like Apple has also acknowledged the issue on the system status page
https://developer.apple.com/system-status/