Avoid dict mutation in VaspInputSet.structure setter (#4520)

`_config_dict["POTCAR"]` was being modified in-place during Yb PSP warning check
this caused `user_potcar_settings` to be permanently merged into  `_config_dict["POTCAR"]`, might lead to incorrect POTCAR settings in subsequent operations

Author: janosh

src/pymatgen/io/vasp/sets.py

@@ -427,7 +427,7 @@ def structure(self, structure: Structure | None) -> None:
                 get_valid_magmom_struct(structure, spin_mode="auto")
 
             struct_has_Yb = any(specie.symbol == "Yb" for site in structure for specie in site.species)
-            potcar_settings = self._config_dict.get("POTCAR", {})
+            potcar_settings = self._config_dict.get("POTCAR", {}).copy()
             if self.user_potcar_settings:
                 potcar_settings.update(self.user_potcar_settings)
             uses_Yb_2_psp = potcar_settings.get("Yb", None) == "Yb_2"