Unhandled exception: System.IO.IOException: The process cannot access the file 'C:\Users\...\AppData\Local\UnrealEngine\Intermediate\Build\BuildRules\MarketplaceRules.dll' because it is being used by another process.

This seems to be an issue 5.1 onwards, where it's storing these temporary files in the LOCALAPPDATA instead of a location that is per-job. Need to figure out if we can override this (or maybe just set the LOCALAPPDATA environment variable before running the UBT commands?)