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

recipe_modules/generator_script/example.py

 # Copyright 2013 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.
 
 from recipe_engine.recipe_api import Property
 
 DEPS = [
   'generator_script',
   'json',
   'path',
   'properties',
   'step',
 ]
 
 PROPERTIES = {
   'script_name': Property(kind=str),
   'script_env': Property(default=None, kind=dict),
 }
 
 def RunSteps(api, script_name, script_env):
-  api.path['checkout'] = api.path['slave_build']
+  api.path['checkout'] = api.path['tmp'].join('checkout')
   script_name = api.properties['script_name']
   script_env = api.properties.get('script_env')
   api.generator_script(script_name, env=script_env)
 
 def GenTests(api):
   yield (
     api.test('basic') +
     api.properties(script_name="bogus") +
     api.generator_script(
       'bogus',
@@ skipping 41 matching lines @@
   )
 
   yield (
     api.test('malformed_command') +
     api.properties(script_name='malformed.py') +
     api.generator_script(
       'malformed.py',
       {'name': 'run', 'cmd': ['echo', 'there are', 4, 'cows']}) +
     api.expect_exception('AssertionError')
   )