I made a look at this and it seems it can not build any kind of font (including variable) without first downloading opensans font source and putting it there.
در چهارشنبه, اوت 30 2023 at ۱۵:۱۵:۴۴ +02:00:00, Bastian Germann <bage@debian.org> نوشته بود:
Please check the latest git commit where I have demonstrated
that you can at least run parts of the build.
You should investigate how to call fontforge to build the ttf files.