gio: Rename GTK_USE_PORTAL to GIO_USE_PORTALS

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 gtk!4829 (merged), so keeping the GTK_USE_PORTAL name in GLib doesn’t make sense anyway.

Signed-off-by: Philip Withnall pwithnall@endlessos.org

Fixes: #3107 (closed)

Closes #3107 (closed)