Honor environment proxy settings

Change-Id: Id38e05863ec014604f47afe94b0bfbf63b0924c9

internal/driver/fetch.go

 	client := &http.Client{
 		Transport: &http.Transport{
 			ResponseHeaderTimeout: timeout + 5*time.Second,
+			Proxy: http.ProxyFromEnvironment,
 		},
 	}
 	return client.Get(url)