Fix #126824: Allow instancing override child collection in outliner

In case of overridden collection, only root collection can be instanced
from outliner. Whereas, "Add collection instance" operation from viewport
works with child collections as well. To make outliner operation consistent,
set `is_liboverride_allowed` to true. This will let instancing of child collection that
are overridden.

Co-authored-by: Bastien Montagne <mont29@noreply.localhost>
Pull Request: https://projects.blender.org/blender/blender/pulls/126881

source/blender/editors/space_outliner/outliner_collections.cc

@@ -796,7 +796,7 @@ static int collection_instance_exec(bContext *C, wmOperator * /*op*/)
   CollectionEditData data{};
   data.scene = scene;
   data.space_outliner = space_outliner;
-  data.is_liboverride_allowed = false; /* No instancing of non-root collections. */
+  data.is_liboverride_allowed = true;
   data.is_liboverride_hierarchy_root_allowed = true;
 
   data.collections_to_edit = BLI_gset_ptr_new(__func__);