go.bbclass: set GOPROXY

go.bbclass: set GOPROXY

The GOPROXY is already correctly defined on the native sys root
and this can be checked using the bitbake devshell:

| $ go env GOPROXY
| https://proxy.golang.org,direct

The go_do_compile task calls the compiler directly so the
GOPROXY env is not seen because it's not defined in the shell.
Defining it explicitly solves this problem and was to avoid
setting it in the recipes itself.

Signed-off-by: Jose Quaresma <jose.quaresma@foundries.io>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>