Revert modified package.json during prepare

The publish step requires a pristine tree, so revert to aim for cleanliness

build.py

         unity_package_build.copy_file_to_project("CHANGELOG.md", ".", package_path, logger)
 
 def cleanup(logger):
-    logger.info("Removing temporary files:")
+    logger.info("Reverting temporary file changes:")
-#        files.append(os.path.join(folder, "package.json"))
+        files.append(os.path.join(folder, "package.json"))
-        os.remove(file)
+        subprocess.call(["git", "checkout", file], cwd=".")
 
 # Prepare an empty project for editor tests
 def prepare_editor_test_project(repo_path, project_path, logger):