nspawn: wait for network namespace creation before interface setup (#8633)

nspawn: wait for network namespace creation before interface setup (#8633)

Otherwise, network interfaces can be "moved" into the container's
namespace while it's still the same as the host namespace, in which case
e.g. host0 for a veth ends up on the host side instead of inside the
container.

Regression introduced in 0441378080489e4ab6704cd0a2d78cb1ceaca899.

Fixes #8599.