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
https://github.com/sdispater/tomlkit/pull/234. This PR corrects the
nested table level, because `preserved` map comes from the poetry config
table.

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

@@ -75,7 +75,7 @@ class SelfCommand(InstallerCommand):
         content = Factory.create_pyproject_from_package(package=package)
         content = Factory.create_pyproject_from_package(package=package)
 
 
         for key in preserved:
         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")
         self.system_pyproject.write_text(content.as_string(), encoding="utf-8")