Makefile: use $(GO) env-var everywhere
Makefile: use $(GO) env-var everywhere

One location was still using the hardcoded default binary.

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

Closes: #960
Approved by: rhatdan

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