allow building with custom Go binary
allow building with custom Go binary

I need to build deb packages using golang-1.10 on Ubuntu Xenial which is
present in a non-default location while /usr/bin/go is at golang-1.6.

With this commit I can simply run:
`GO=/usr/lib/go-1.10/bin/go ./build.sh`
to use golang-1.10.

Signed-off-by: Lokesh Mandvekar <46b23e80209bb43857d25fd87bafe9be89c7d33a@fedoraproject.org>

containernetworking/plugins,containernetworking/plugins