presubmit: use depot_tools.on_path rather than a handrolled equivalent.

Noticed this while attempting to debug issues with the
src-internal-presubmit builder. Not sure if this is the cause but
would like to at least rule it out.

Bug: 925774
Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: build_limited_scripts_slave
Recipe-Nontrivial-Roll: release_scripts
Change-Id: I323f49e23d2c8af11b374e22ebfbcf5bcd836ba3
Reviewed-on: https://chromium-review.googlesource.com/c/1464049
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>

recipes/README.recipes.md

@@ -723,7 +723,7 @@ Raises:
     StepFailure or InfraFailure.
 ### *recipe_modules* / [presubmit](/recipes/recipe_modules/presubmit)
 
-[DEPS](/recipes/recipe_modules/presubmit/__init__.py#1): [recipe\_engine/context][recipe_engine/recipe_modules/context], [recipe\_engine/path][recipe_engine/recipe_modules/path], [recipe\_engine/python][recipe_engine/recipe_modules/python], [recipe\_engine/step][recipe_engine/recipe_modules/step]
+[DEPS](/recipes/recipe_modules/presubmit/__init__.py#1): [depot\_tools](#recipe_modules-depot_tools), [recipe\_engine/context][recipe_engine/recipe_modules/context], [recipe\_engine/path][recipe_engine/recipe_modules/path], [recipe\_engine/python][recipe_engine/recipe_modules/python], [recipe\_engine/step][recipe_engine/recipe_modules/step]
 
 #### **class [PresubmitApi](/recipes/recipe_modules/presubmit/api.py#7)([RecipeApi][recipe_engine/wkt/RecipeApi]):**
 

recipes/recipe_modules/presubmit/__init__.py

 DEPS = [
+  'depot_tools',
   'recipe_engine/context',
   'recipe_engine/path',
   'recipe_engine/python',

recipes/recipe_modules/presubmit/api.py

@@ -14,11 +14,6 @@ class PresubmitApi(recipe_api.RecipeApi):
 
     name = kwargs.pop('name', 'presubmit')
 
-    env = self.m.context.env
-    env.setdefault('PATH', '%(PATH)s')
-    env['PATH'] = self.m.path.pathsep.join([
-        env['PATH'], str(self._module.PACKAGE_REPO_ROOT)])
-
-    with self.m.context(env=env):
+    with self.m.depot_tools.on_path():
       return self.m.python(
           name, self.presubmit_support_path, list(args), **kwargs)

recipes/recipe_modules/presubmit/examples/full.expected/basic.json

@@ -5,8 +5,10 @@
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/presubmit_support.py"
     ],
-    "env": {
-      "PATH": "<PATH>:RECIPE_PACKAGE_REPO[depot_tools]"
+    "env_prefixes": {
+      "PATH": [
+        "RECIPE_PACKAGE_REPO[depot_tools]"
+      ]
     },
     "name": "presubmit"
   },