fix: compile all plugins

[thomasferrandiz]

xx-go --wrap is not compatible with the build_linux.sh script so the plugins were not actually built.

Related issue: #48

[manuelbuil]

Can you briefly explain in the description how you are fixing the problem?

[thomasferrandiz]

The helper script ``xx-gohas 2 modes. The first one is just a wrapper forgo` which sets the right var like `GOOS` to cross-compile properly to `TARGETPLATFORM`.
The second mode (`xx-go --wrap`) changes the GOENV file so that all calls to `go` are set to cross-compile.
I don't know why but this 2nd mode is not compatible which the script `build_linux.sh` from the upstream cni repo.

So I use the 1st mode by calling the build_linux.sh script with the GO var set to xx-go so that alls calls to ${GO:-go} build in the script are set-up to cross-compile when needed.