Update cross_compiling.md

I am running the build of Go we built in your workshop a couple of months ago. I discovered that although `go env` thinks that `CGO_ENABLED=1` it's not actually in the build, which ends up with some oddity. It turns out that we disabled it when building the cross-compiling tools.  This PR adds a little note to explain how to not end up like this when building the cross-compiling tools.

Thanks again!