cmd/go/internal/envcmd: fix showing GODEBUG env value

It was accidently dropped in CL 563137

For #34208

Change-Id: I0aea93fda6260f07bc662dea92d1eabb26f82ca0
Reviewed-on: https://go-review.googlesource.com/c/go/+/586095
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>

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

@@ -109,7 +109,7 @@ func MkEnv() []cfg.EnvVar {
 		{Name: "GOTOOLDIR", Value: build.ToolDir},
 		{Name: "GOVCS", Value: cfg.GOVCS},
 		{Name: "GOVERSION", Value: runtime.Version()},
-		{Name: "GODEBUG"},
+		{Name: "GODEBUG", Value: os.Getenv("GODEBUG")},
 	}
 
 	for i := range env {