CI: allow Go 1.13 for Docker/Moby compatibility
CI: allow Go 1.13 for Docker/Moby compatibility

Docker/Moby still uses Go 1.13 for building containerd binaries.

Signed-off-by: Akihiro Suda <10ddef0dea2a66ab193b07198faa879adcb213d8@hco.ntt.co.jp>

containerd/containerd,vdemeester/containerd,containerd/containerd,mikebrow/containerd,thaJeztah/containerd,vdemeester/containerd,estesp/containerd,estesp/containerd,dmcgowan/containerd,mikebrow/containerd,dmcgowan/containerd,thaJeztah/containerd