Clean up some genrule() script code

The XJC genrule in particular was dirtying /tmp which I believe may have
been causing problems for Donuts.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129451600

java/google/registry/xjc/list_generated_files.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 # Copyright 2016 The Domain Registry Authors. All Rights Reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");