Bluetooth: Mesh: Store network at the end of provisioning

After #31176, the network would get stored immediately in
bt_mesh_net_create, causing the address and devicekey to get stored as
their zero-initialized version, as they're only being set in the
bt_mesh_comp_provision call, which fires after.

Fixes #32067.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>