recipe engine: extract infra-specific paths out of the engine

recipe_modules/json/example.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 DEPS = [
   'json',
   'path',
   'python',
   'raw_io',
   'step',
@@ skipping 35 matching lines @@
   example_dict = {'x': 1, 'y': 2}
 
   # json.input(json_data) expands to a path containing that rendered json
   step_result = api.step('json through',
     ['cat', api.json.input(example_dict)],
     stdout=api.json.output(),
     step_test_data=lambda: api.json.test_api.output_stream(example_dict))
   assert step_result.stdout == example_dict
 
   # json.read reads a file containing json data.
-  leak_path = api.path['slave_build'].join('temp.json')
+  leak_path = api.path['tmp'].join('temp.json')
   api.step('write json to file',
     ['cat', api.json.input(example_dict)],
     stdout=api.raw_io.output(leak_to=leak_path))
   step_result = api.json.read(
       'read json from file we just wrote', leak_path,
       step_test_data=lambda: api.json.test_api.output(example_dict))
   assert step_result.json.output == example_dict
 
 
 def GenTests(api):
   yield (api.test('basic') +
       api.step_data('echo1', stdout=api.json.output([1, 2, 3])) +
       api.step_data(
           'foo',
           api.json.output([1, 2, 3], name='1') +
           api.json.output(['x', 'y', FULLWIDTH_Z], name='2'),
       ))