nspawn: join network namespace before cloning user namespace

nspawn: join network namespace before cloning user namespace

When both --private-users and --network-namespace-path are
specified, systemd-nspawn fails to start with "Operation not
permitted" during the setns() call.

This occurs because of the following execution sequence:
1. The outer child calls raw_clone() with CLONE_NEWUSER to create
   the new user namespace.
2. The inner child is spawned inside this new user namespace.
3. The inner child then attempts to call setns() to join the
   external network namespace.

Because the inner child is already running inside
the restricted user namespace, the kernel rejects
the setns() call to join a network namespace owned by
a different (host/more privileged) user namespace.

Fix this by moving the setns() call to the outer child, executing it
just before the raw_clone() call. This ensures the network namespace
is joined while the process still has the necessary privileges, which
also aligns with the inner child's expectation that the network
namespace is already set up upon entry.