cmd/go: fix incorrect determining default value of CGO_ENABLED

The default value is the value obtained when
no environment variables are set and go env  -w is not used.

In the past,
we used the current value
(may be modified by an environment variable to a non-default value),
error was used as the default value.

For #69994

Change-Id: Iead3a6cacd04dc51a094ffb9f7bb7553320fcd78
Reviewed-on: https://go-review.googlesource.com/c/go/+/621995


Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>