build: Reuse schemas and GIO modules paths variables

meson's post install script, compiles glib schemas and creates a
cache from the GIO modules. To do this, an assumption is made
regarding the directories where the files are installed under
prefix.

However, the final directories are available as meson variables
and these can be passed to the post install script, so there is
no need to make any assumption.

https://bugzilla.gnome.org/show_bug.cgi?id=794365