Vagrant virtualbox worker env override fixes

* Change ENV var check to only validate the existence of the
  var in the local env
* Add VAGRANT_WORKER_SCALE env variable override to control
  GUI scaling for virtualbox

Signed-off-by: James W. Brinkerhoff <jwb@paravolve.net>

test/_vagrant/vagrant_test.go

@@ -95,7 +95,6 @@ func TestVagrantSetupGuide(t *testing.T) {
 
 	t.Logf("WorkflowID: %s", workflowID)
 
-	os.Setenv("VAGRANT_WORKER_GUI", "false")
 	worker, err := vagrant.Up(ctx,
 		vagrant.WithLogger(t.Logf),
 		vagrant.WithMachineName("worker"),