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>

src/cmd/go/go_test.go

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