cmd/go: use ProxyFromEnvironment in -insecure mode also

Be consistent on whether the http proxy environment variables are
respected regardless of whether -insecure is used.

Updates #18519

Change-Id: Ib157eaacfd342dd3bfcd03e64da18c98c609cae3
Reviewed-on: https://go-review.googlesource.com/34818
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/http.go

@@ -33,6 +33,7 @@ var httpClient = http.DefaultClient
 var impatientInsecureHTTPClient = &http.Client{
 	Timeout: 5 * time.Second,
 	Transport: &http.Transport{
+		Proxy: http.ProxyFromEnvironment,
 		TLSClientConfig: &tls.Config{
 			InsecureSkipVerify: true,
 		},