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>