net/v2ray: Switch to USES=go:modules, fix build with go1.13

> Building for v2ray-4.20.0

go: github.com/golang/mock@v1.2.0: Get https://proxy.golang.org/github.com/golang/mock/@v/v1.2.0.mod: dial tcp: lookup proxy.golang.org on 209.222.18.218:53: write udp 127.0.0.1:43894->209.222.18.218:53: write: permission denied

• Error code 1

PR: 240282
Submitted by: Dmitri Goutnik <dg@syrec.org>
Approved by: Yanhui Shen <shen.elf@gmail.com> (maintainer)
Pointy hat: jlaffaye