Makefile: allow overriding go command by environment
Makefile: allow overriding go command by environment

This is required for environments/build systems where a specific
go version / command needs to be used.

Signed-off-by: Enrico Weigelt, metux IT consult <59bd0a3ff43b32849b319e645d4798d8a5d1e889@metux.net>

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