use Makefile var for go compiler
use Makefile var for go compiler

This will allow compilation with a custom go binary,
for example /usr/lib/go-1.8/bin/go instead of /usr/bin/go on Ubuntu
16.04 which is still version 1.6

Signed-off-by: Lokesh Mandvekar <lsm5@fedoraproject.org>

Closes: #278
Approved by: rhatdan

TomSweeneyRedHat/buildah,nalind/buildah-1,nalind/buildah,ipbabble/buildah,TomSweeneyRedHat/buildah,nalind/buildah-1,projectatomic/buildah,nalind/buildah-1,projectatomic/buildah,ipbabble/buildah,projectatomic/buildah,rhatdan/buildah,rhatdan/buildah,rhatdan/buildah,nalind/buildah,TomSweeneyRedHat/buildah