nspawn: ensure single-process container running as --user can access credentials

nspawn: ensure single-process container running as --user can access credentials

When starting a container with --user, the new uid will be resolved and switched to
only in the inner child, at the end of the setup, by spawning getent. But the
credentials are set up in the outer child, long before the user is resolvable,
and the directories/files are made only readable by root and read-only, which
means they cannot be changed later and made visible to the user.

When this particular combination is specified, it is obvious the caller wants
the single-process container to be able to use credentials, so make them world
readable only in that specific case.

Fixes https://github.com/systemd/systemd/issues/31794