net/nats-streaming-server: Unbreak with go1.13

> Building for nats-streaming-server-0.15.0

go: github.com/go-sql-driver/mysql@v1.4.1: Get https://proxy.golang.org/github.com/go-sql-driver/mysql/@v/v1.4.1.mod: dial tcp: lookup proxy.golang.org on 209.222.18.218:53: write udp 127.0.0.1:51039->209.222.18.218:53: write: permission denied

• Error code 1

• Switch to USES=go:modules
• Remove GO_PKGNAME, not needed with modules

PR: 240314
Submitted by: Dmitri Goutnik <dg@syrec.org>
Approved by: olgeni (maintainer)
Pointy hat: jlaffaye