[context] Split "env" values and prefixes.

recipe_modules/step/api.py

Index: recipe_modules/step/api.py
diff --git a/recipe_modules/step/api.py b/recipe_modules/step/api.py
index 4a250c1b057290e7cebcc247be4c41c866297e16..9e831f75d97fb3b46e8cfcfba29894e050eb88ea 100644
--- a/recipe_modules/step/api.py
+++ b/recipe_modules/step/api.py
@@ -187,12 +187,11 @@ class StepApi(recipe_api.RecipeApiPlain):
     cwd = self.m.context.cwd
     if cwd is not None and cwd != self.m.path['start_dir']:
       kwargs['cwd'] = cwd
+
+    with self.m.context(env_prefixes={'PATH': self._prefix_path}):
+      kwargs['env_prefixes'] = self.m.context.env_prefixes
     kwargs['env'] = self.m.context.env
-    if self._prefix_path:
-      ps = self.m.path.pathsep
-      prefix = ps.join(self._prefix_path)
-      suffix = kwargs['env'].get('PATH', '%(PATH)s')
-      kwargs['env']['PATH'] = '%s%s%s' % (prefix, ps, suffix)
+    kwargs['pathsep'] = self.m.path.pathsep
     kwargs['infra_step'] = self.m.context.infra_step or bool(infra_step)
     kwargs['step_nest_level'] = self.m.context.nest_level
     kwargs['name'] = full_name