util: Remove undesired variables from activation environment

Commit 646b9bc (included in 3.33.92) prevented one run of gnome-session
from uploading environment variables into `systemd --user` that should not
leak into a different login session, such as XDG_SESSION_ID. However,
non-GNOME session managers (and in particular the forks of gnome-session
found in Cinnamon and MATE) might still upload those environment variables.

The other session managers should be fixed, similar to 646b9bc, but we
can mitigate this for GNOME sessions by actively unsetting the undesired
variables, instead of just not setting them.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Resolves: https://gitlab.gnome.org/GNOME/gnome-session/-/issues/86

gnome-session/gsm-util.c

@@ -646,6 +646,8 @@ gsm_util_export_user_environment (GError     **error)
         g_variant_builder_open (&builder, G_VARIANT_TYPE ("as"));
         for (i = 0; variable_unsetlist[i] != NULL; i++)
                 g_variant_builder_add (&builder, "s", variable_unsetlist[i]);
+        for (i = 0; variable_blacklist[i] != NULL; i++)
+                g_variant_builder_add (&builder, "s", variable_blacklist[i]);
         g_variant_builder_close (&builder);
 
         g_variant_builder_open (&builder, G_VARIANT_TYPE ("as"));