reprotest fails chown on disorderfs

reprotest uses disorderfs for the second build. Unless one passes --multi-user=yes to disorderfs, it doesn't work with multiple users. When a package build does chown, the build fails with -EINVAL.

I'm not sure whether this is a salsa-ci issue or a reprotest issue. Filing here initially, please help with directing it to the proper place. I imagine that possibly, we need --multi-user=yes here. Alternatively, we might need -rfakeroot (despite running the build as root). Any other ideas?