go.tools/dashboard/builder: respect CGO_ENABLED value from the environment

Allow builder owners to set CGO_ENABLED to override the logic in cmd/go

LGTM=minux.ma, adg
R=golang-codereviews, minux.ma, adg
CC=golang-codereviews
https://golang.org/cl/63570046

dashboard/builder/env.go

@@ -20,6 +20,7 @@ import (
 // to the envv of its subprocesses.
 var extraEnv = []string{
 	"GOARM",
+	"CGO_ENABLED",
 
 	// For Unix derivatives.
 	"CC",