cmd/go: clear GOPROXY in TestGoGetInsecure

TestGoGetInsecure verifies that 'go get -insecure' can fetch a
particular package. However, the GOPROXY protocol does not provide a
means for proxies to indicate packages as insecure; thus, proxies
cannot safely serve those packages.

Updates #30571

Change-Id: I447776dff98bd8ee6eb5055b897b9c7d293e3423
Reviewed-on: https://go-review.googlesource.com/c/go/+/165745
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -3670,6 +3670,7 @@ func TestGoGetInsecure(t *testing.T) {
 			tg.tempFile("go.mod", "module m")
 			tg.cd(tg.path("."))
 			tg.setenv("GO111MODULE", "on")
+			tg.setenv("GO111PROXY", "")
 		} else {
 			tg.setenv("GOPATH", tg.path("."))
 			tg.setenv("GO111MODULE", "off")