With iCloud, folders you think are “local only” may also be shared. That’s the case of the Documents folder. Hence the advices given by other users.
Well, the issue is specific to Sonoma. Does not happen with Ventura and earlier, I changed nothing apart from the OS, and I can roll back to Ventura and it doesn’t happen.
The other option is to create a .nosync file in the project folder before building, and then remove it afterwards. touch .nosync achieves this from the command line. This could be part of a build script.