cmd/dist: forward stderr if 'go env CGO_ENABLED' fails

The default error string for a command failure is just its status code,
and "exit status 1" is not at all helpful for debugging.

Change-Id: I822c89bcc9e73283b33e01792bf9c40b1add3c35
Reviewed-on: https://go-review.googlesource.com/c/go/+/222308


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>