fix: wrong nested table level when overwriting pyproject (#6512)

Previously, we rely on the `display_name` stored with `tomlkit.Table`,
which is an internal implementation detail, and it breaks
python-poetry/tomlkit#234. This PR corrects the
nested table level, because `preserved` map comes from the poetry config
table.

(cherry picked from commit 20352b6)

src/poetry/console/commands/self/self_command.py

@@ -75,7 +75,7 @@ def generate_system_pyproject(self) -> None:
         content = Factory.create_pyproject_from_package(package=package)
 
         for key in preserved:
-            content[key] = preserved[key]
+            content["tool"]["poetry"][key] = preserved[key]  # type: ignore[index]
 
         self.system_pyproject.write_text(content.as_string(), encoding="utf-8")