Source public.skipExportGlyphs also from <variable-font>'s <lib> element

[madig]

Designspaces v5 may need to define different skip lists for different variable fonts, so when compiling a variable font, ufo2ft should look first at <variable-font>'s lib and then at the global Designspace lib.

[ryanbugden]

I second this, for the reasons above.
public.skipExportGlyphs is only supported in the global lib, not individual variable-font libs.