Would you please add mirror support for go binary download

[liaoya]

Now the default go binary download url is https://dl.google.com/go, can you provide a environment variable to allow override like

echo "https://dl.google.com/go/go${version}.${os}-${arch}.tar.gz" => echo "${GO_BINARY_BASE_URL:-https://dl.google.com/go}/go${version}.${os}-${arch}.tar.gz".

moovweb/gvm#237 did the similar work.