arm64 is listed as supported system-agent install but arm64 k3s binary is unavailable

[bgulla]

# setup_arch set arch and suffix,
# fatal if architecture not supported.
setup_arch() {
    case ${ARCH:=$(uname -m)} in
    amd64)
        ARCH=amd64
        SUFFIX=$(uname -s | tr '[:upper:]' '[:lower:]')-${ARCH}
        ;;
    x86_64)
        ARCH=amd64
        SUFFIX=$(uname -s | tr '[:upper:]' '[:lower:]')-${ARCH}
        ;;
    arm64)
        ARCH=arm64
        SUFFIX=-${ARCH}
        ;;
    aarch64)
        ARCH=arm64
        SUFFIX=-${ARCH}
        ;;
    arm*)
        ARCH=arm
        SUFFIX=-${ARCH}hf
        ;;
    *)
        fatal "unsupported architecture ${ARCH}"
        ;;
    esac
}

Lists arm64 as 'supported' but the install will fail.

system-agent/examples/install-k3s/install-k3s.plan is the image used to install the base k3s binary but a corresponding arm64 image does not exist in the manifest.

if we plan to not build arm64 k3s plans, we should mark the arch as disabled in rancher-system-agent install script.