node_installer(k3s): support custom systemd service name

[vdice]

The canonical k3s installer script supports customized naming of the k3s systemd service:

#   - INSTALL_K3S_NAME
#     Name of systemd service to create, will default from the k3s exec command
#     if not specified. If specified the name will be prefixed with 'k3s-'.

This issue is to allow RCM operators/installers to provide a customized k3s service name to be restarted after a shim is installed (see https://github.com/spinframework/runtime-class-manager/blob/main/internal/containerd/restart_unix.go).