Feature file include paths differ depending on whether we run from UFOs or from Glyphs/DS

I just found that when I take e.g. https://gitlab.gnome.org/GNOME/cantarell-fonts/tree/ufo-conversion/src and extract the feature file into a shared `family.fea` next to the Designspace file, I need to use `include(family.fea)` in the UFOs when using `fontmake -m` and change it to `include(../family.fea)` when using `fontmake -u`.