The 10-containerd-net.conflist file generated from the conf_template
should be written atomically so that partial writes are not visible to
CNI plugins. Use the new consistentfile package to ensure this on
Unix-like platforms such as Linux, FreeBSD, and Darwin.
Fixes https://github.com/containerd/containerd/issues/8607
Signed-off-by: Samuel Karp <samuelkarp@google.com>