There's a merged pull request on the void-packages repository

xbps-src: forbid use with root.
https://github.com/void-linux/void-packages/pull/18807

Description:
There's no reason to use xbps-src as root, and looks like
there's a lot of reports of this abuse.

Unless XBPS_ALLOW_CHROOT_BREAKOUT is set (travis),
using xbps-src as root now returns an error.