net: make GODEBUG=netdns=cgo force cgo as documented

It wasn't working. The wrong variable was used.

This would ideally have tests. It's also DEBUG.

Fixes #11816

Change-Id: Iec42d229b81d78cece4ba5c73f3040e2356eb98f
Reviewed-on: https://go-review.googlesource.com/12544
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/conf.go

@@ -80,7 +80,7 @@ func initConfVal() {
 	_, localDomainDefined := syscall.Getenv("LOCALDOMAIN")
 	if os.Getenv("RES_OPTIONS") != "" ||
 		os.Getenv("HOSTALIASES") != "" ||
-		netCgo ||
+		confVal.netCgo ||
 		localDomainDefined {
 		confVal.forceCgoLookupHost = true
 		return