Makefile: use go proxy
Makefile: use go proxy

Use GOPROXY=https://proxy.golang.org to speed up fetching dependencies.
Setting it makes `make vendor` ten times faster in my local env.

For details please refer to https://proxy.golang.org/.

Signed-off-by: Valentin Rothberg <c8d7c67387706f1534261b1aba83f5c355c00ff5@redhat.com>

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