builder/vmware/common: fix missing format argument for Fatalf call in test

Fixes the following vet report:

builder/vmware/common/step_shutdown_test.go:130: missing argument for Fatalf("%s"): format reads arg 1, have only 0 args

builder/vmware/common/step_shutdown_test.go

@@ -127,7 +127,7 @@ func TestStepShutdown_locks(t *testing.T) {
 	lockPath := filepath.Join(dir.dir, "nope.lck")
 	err := ioutil.WriteFile(lockPath, []byte("foo"), 0644)
 	if err != nil {
-		t.Fatalf("err: %s")
+		t.Fatalf("err: %s", err)
 	}
 
 	// Remove the lock file after a certain time