Fix k3os bug with wrong arch

Wrong arch was being set from refactor:
Fixes: https://github.com/netbootxyz/netboot.xyz/issues/1002

roles/netbootxyz/templates/menu/k3os.ipxe.j2

@@ -9,7 +9,7 @@ goto ${menu} ||
 :k3os
 set os {{ releases.k3os.name }}
 set os_arch ${arch}
-iseq ${os_arch} x86_64 && set os_arch x86_64 ||
+iseq ${os_arch} x86_64 && set os_arch amd64 ||
 set k3os_mirror {{ releases.k3os.mirror }}
 isset ${k3os_version} || set k3os_version latest
 isset ${k3os_install_device} || set k3os_install_device /dev/sda