Roll recipe dependencies (trivial).

This is an automated CL created by the recipe roller. This CL rolls
recipe changes from upstream projects (recipe_engine) into this repository.

The build that created this CL was
https://ci.chromium.org/b/8749197358679417393

recipe_engine:
https://chromium.googlesource.com/infra/luci/recipes-py/+/c406446548974dc887dac0a676838e08e09b526a
  c406446 (iannucci@chromium.org)
      [recipes.py] Directly set PYTHONPATH to the engine path.

Please check the following references for more information:
- autoroller, https://chromium.googlesource.com/infra/luci/recipes-py/+/main/doc/workflow.md#autoroller
- rollback, https://chromium.googlesource.com/infra/luci/recipes-py/+/main/doc/workflow.md#rollback
- cross-repo dependencies, https://chromium.googlesource.com/infra/luci/recipes-py/+/main/doc/cross_repo.md

Use https://goo.gl/noib3a to file a bug.

Recipe-Tryjob-Bypass-Reason: Autoroller
Ignore-Freeze: Autoroller
Bugdroid-Send-Email: False
Change-Id: Id0eae4a2507e1988bc48b34b21ebadf3690e7456
Reviewed-on: https://fuchsia-review.googlesource.com/c/infra/recipes/+/1040163
Commit-Queue: GI Roller <global-integration-roller@fuchsia-infra.iam.gserviceaccount.com>
diff --git a/infra/config/recipes.cfg b/infra/config/recipes.cfg
index cd9afd2..cee3491 100644
--- a/infra/config/recipes.cfg
+++ b/infra/config/recipes.cfg
@@ -18,7 +18,7 @@
   "deps": {
     "recipe_engine": {
       "branch": "refs/heads/main",
-      "revision": "eb434718b27659fb36792b710cf2b80b8d85aa0b",
+      "revision": "c406446548974dc887dac0a676838e08e09b526a",
       "url": "https://chromium.googlesource.com/infra/luci/recipes-py"
     }
   },
diff --git a/recipes.py b/recipes.py
index 05ca941..c6ef83a 100755
--- a/recipes.py
+++ b/recipes.py
@@ -235,13 +235,10 @@
   if not shutil.which(vpython):
     return f'Required binary is not found on PATH: {vpython}'
 
-  # We unset PYTHONPATH here in case the user has conflicting environmental
-  # things we don't want them to leak through into the recipe_engine which
-  # manages its environment entirely via vpython.
-  #
-  # NOTE: os.unsetenv unhelpfully doesn't exist on all platforms until python3.9
-  # so we have to use the cutesy `pop` formulation below until then...
-  os.environ.pop("PYTHONPATH", None)
+  # We overwrite PYTHONPATH here on purpose; We don't want any conflicting
+  # environmental path leaking through into the recipe_engine which manages its
+  # environment entirely via vpython.
+  os.environ['PYTHONPATH'] = engine_path
 
   spec = '.vpython3'
   debugger = os.environ.get('RECIPE_DEBUGGER', '')