[context] Split "env" values and prefixes.

recipe_modules/step/api.py

 # Copyright 2013 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 import contextlib
 import copy
 
 from recipe_engine import recipe_api
 
 
@@ skipping 169 matching lines @@
         break
 
       step_count = self._step_names.setdefault(full_name, 1) + 1
       self._step_names[full_name] = step_count
       name_suffix = ' (%d)' % step_count
     self._seen_steps.add(full_name)
 
     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['infra_step'] = self.m.context.infra_step or bool(infra_step)
     kwargs['step_nest_level'] = self.m.context.nest_level
     kwargs['name'] = full_name
     kwargs['base_name'] = name
 
     schema = self.make_config()
     schema.set_val(kwargs)
     return self.run_from_dict(schema.as_jsonish())
 
   # TODO(martiniss) delete, and make generator_script use **kwargs on step()
   @recipe_api.composite_step
   def run_from_dict(self, dct):
     return self.step_client.run_step(dct)