Hooray! My "python-pivy" patches were merged by Guillaume Le Vaillant:
https://git.savannah.gnu.org/cgit/guix.git/commit/?id=75f3cca50a4c53c65aa6b424276acdf023d51008
Now FreeCAD can be build with Guix again and its "Draft" workbench should be working properly.
@avp Very cool! FreeCAD guy here, I've been experimenting with Guix myself lately, it's awesome. `guix system vm`, `guix home container` etc are amazing tools!