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

export XBPS_BINPKG_EXISTS rather than mark it as readonly
https://github.com/void-linux/void-packages/pull/11874
Description: srcpkg_get_version and srcpkg_get_pkgver require unsetting
XBPS_BINPKG_EXISTS to work properly (e.g. without it, some packages have
a trailing -).  Unfortunately, bash does not allow unsetting readonly
variables, which XBPS_BINPKG_EXISTS is exported as in xbps-src.

To resolve this problem, simply export XBPS_BINPKG_EXISTS as a normal
variable rather than marking it as readonly.  This addresses #11740.