When compiling a variable font from a Designspace, an instance's "com.schriftgestaltung.export" lib key should be honored

In Glyphs, an instance can be disabled. This is saved in the instance's `com.schriftgestaltung.export` lib key. fontmake should honor this lib key, even when running from a Designspace instead of a Glyphs file.

Example: https://gitlab.gnome.org/GNOME/cantarell-fonts/blob/ufo-conversion/src/Cantarell.designspace