command/build,validate: compiles again with latest user var changes

command/build/command.go

@@ -150,7 +150,7 @@ func (c Command) Run(env packer.Environment, args []string) int {
 		log.Printf("Preparing build: %s", b.Name())
 		b.SetDebug(cfgDebug)
 		b.SetForce(cfgForce)
-		err := b.Prepare()
+		err := b.Prepare(nil)
 		if err != nil {
 			env.Ui().Error(err.Error())
 			return 1

command/validate/command.go

@@ -79,7 +79,7 @@ func (c Command) Run(env packer.Environment, args []string) int {
 	// Check the configuration of all builds
 	for _, b := range builds {
 		log.Printf("Preparing build: %s", b.Name())
-		err := b.Prepare()
+		err := b.Prepare(nil)
 		if err != nil {
 			errs = append(errs, fmt.Errorf("Errors validating build '%s'. %s", b.Name(), err))
 		}