[release-branch.go1.12] cmd/go: clear GOPROXY in TestGoGetInsecure

[release-branch.go1.12] 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.

This also squashes the typo fix from CL 167086.

Updates #30571
Fixes #33758

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>
Reviewed-on: https://go-review.googlesource.com/c/go/+/199820
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>