Fix #127611: Linked material can be modified by Copy/Paste Material

Seems like we can use `object_materials_supported_poll` (similar to
other material operators) that already checks the right things.

Pull Request: https://projects.blender.org/blender/blender/pulls/128996

source/blender/editors/render/render_shading.cc

@@ -2854,6 +2854,7 @@ void MATERIAL_OT_paste(wmOperatorType *ot)
 
   /* api callbacks */
   ot->exec = paste_material_exec;
+  ot->poll = object_materials_supported_poll;
 
   /* flags */
   ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO | OPTYPE_INTERNAL;