When building the windows plugin exe's (host-local, flannel, win-overlay, win-bridge),
it was necessary to use 'GOOS=windows go build path/to/plugin' rather than the build script.
This makes 'GOOS=windows GOARCH=amd64 ./build.sh' build all the windows plugin binaries.
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 <lsm5@fedoraproject.org>