Hey, I've submitted a merge request in salsa to address this issue.The FTBFS is fixed in a newer version of mkfontscale, so I'm proposing to update it. Because mkfontdir is now bundled in the newer version of mkfontscale, we need to drop the mkfontdir tarball.
https://salsa.debian.org/xorg-team/font/xfonts-utils/-/merge_requests/2