The `GTK_USE_PORTAL` environment variable has started to be misused by
users, which is causing deployment issues (such as portal services
themselves ending up being forced to use portals, which is never going
to work).
Try and sidestep users’ broken configurations by renaming the
environment variable, and also separating it from the old GTK
environment variable, since the GLib one affects a lot more processes.
This environment variable is meant to be used for
debugging and development, and never in production.
GTK already renamed their environment variable in
https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/4829, so keeping the
`GTK_USE_PORTAL` name in GLib doesn’t make sense anyway.
Signed-off-by: Philip Withnall <pwithnall@endlessos.org>
Fixes: #3107
Test all the snap cases and the unknown sandbox one.
We need to use different test processes as we initialize the portal
type early enough that it can't be changed later.
