cmd/go: fix typo in GoGetInsecure to actually set GOPROXY

I typo'd this variable in CL 165745, and neither I, the reviewer, nor the TryBots noticed.
But the longtest builder noticed, and it's not happy about it.

Updates #30571

Change-Id: I5e3d267346407855ec0d1f340a72dc2c521ecc63
Reviewed-on: https://go-review.googlesource.com/c/go/+/167086


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>