internal/envcmd: print GO111MODULE when executing "go env"

If we look at the issues in the past releases that are related
to go command that involved modules, its usually mention or ask about
the value of GO111MODULE, either in separate line or in separate
comment.

There are quite long time range before GO111MODULE will be removed
(unused).  The next release is still default to auto [1], and until Go
1.13 unsupported (two releases after that) there is about one and half
years after that.

Since the change is not that big (one line) [2], maybe temporary adding
it to "go env" give more clarity and benefit in issue reporting rather
than not.

[1] https://github.com/golang/go/issues/31857

Fixes #29656

Change-Id: I609ad6664774018e4f4147ec6158485172968e16
Reviewed-on: https://go-review.googlesource.com/c/go/+/176837
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/envcmd/env.go

@@ -66,6 +66,7 @@ func MkEnv() []cfg.EnvVar {
 
 	envFile, _ := cfg.EnvFile()
 	env := []cfg.EnvVar{
+		{Name: "GO111MODULE", Value: cfg.Getenv("GO111MODULE")},
 		{Name: "GOARCH", Value: cfg.Goarch},
 		{Name: "GOBIN", Value: cfg.GOBIN},
 		{Name: "GOCACHE", Value: cache.DefaultDir()},