Fix cp command in install-cni.sh
Fix cp command in install-cni.sh

The `cp -n` flag does not exist in BusyBox.  Switched to using a manual
file check and copy if it does not exist.

projectcalico/calico-cni,projectcalico/calico-rkt,projectcalico/calico-cni,projectcalico/calico-cni