bot_update: add depot_tools to PATH so that e.g. apply_issue can be found (reland #1)

recipe_modules/bot_update/api.py

Index: recipe_modules/bot_update/api.py
diff --git a/recipe_modules/bot_update/api.py b/recipe_modules/bot_update/api.py
index c077ae90414f2158cf6d873fccdfec8ea95e8e6f..4b870a4b7eef2d5e2fb47b8df480a9459b1a27f2 100644
--- a/recipe_modules/bot_update/api.py
+++ b/recipe_modules/bot_update/api.py
@@ -57,6 +57,10 @@ class BotUpdateApi(recipe_api.RecipeApi):
     assert isinstance(cmd, (list, tuple))
     bot_update_path = self.resource('bot_update.py')
     kwargs.setdefault('infra_step', True)
+    kwargs.setdefault('env', {})
+    kwargs['env'].setdefault('PATH', '%(PATH)s')
+    kwargs['env']['PATH'] = self.m.path.pathsep.join([
+        kwargs['env']['PATH'], str(self._module.PACKAGE_DIRECTORY)])
     return self.m.python(name, bot_update_path, cmd, **kwargs)
 
   @property