Use shared ccache directory instead of creating .flatpak-builder in projects
This is holdover from https://bugzilla.gnome.org/show_bug.cgi?id=780153. We no longer want to use a flatpak-builder directory in the project, but instead one within the shared state dir, ~/.cache/gnome-builder/flatpak-builder/ccache
(or appropriate g_get_user_cache_dir()
).