The `systemd-nspawn` manpage says:
The `--private-users-ownership=auto` option is implied if
`--private-users=pick` is used.
This is true for the CLI, but not when set in a `.nspawn` file.
Assuming these defaults should be the same across both.
This default was also not updated when `managed` was added by #35685.
As a result, this is a change in behavior for users who didn't set an
explicit `PrivateUsersOwnership=` value in their `.nspawn` file.
