environment: Define promisify of g_query_info_async in the global scope

This is something that will be used in other places outside the
background code, so let's just define it globally without having to care
about the importing order.

Co-authored-by: Marco Trevisan (Treviño) <mail@3v1n0.net>
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2638>