Use GOPROXY to speed up builds (#7984)

Read more here https://proxy.golang.org proposal 
for go1.13

buildscripts/cross-compile.sh

@@ -23,6 +23,7 @@ function _build() {
     export GOOS=$os
     export GOARCH=$arch
     export GO111MODULE=on
+    export GOPROXY=https://proxy.golang.org
     go build -tags kqueue -o /dev/null
 }