This patch adds the OpenLab CI configuration to enable
the support for arm build in OpenLab.
After this, each pull request in containerd will trigger the
containerd-arm64-build job which verified the arm build
on OpenLab ARM cluster.
Related: https://github.com/containerd/containerd/issues/2901
Signed-off-by: Yikun Jiang <yikunkero@gmail.com>