Install cni-plugins package

The RD edition installs the Alpine cni-plugins package instead of using the nerdctl plugins because it needs flannel and loopback plugins too, which are not included in the nerdctl tarball.

The isolation plugin is still installed from nerdctl because it is not included in cni-plugins.

This also changes the installation directory to /usr/libexec/cni (the default for the cni-plugins package), when LIMA_INIT_INSTALL_CNI_PLUGINS is true).