[roll third_party/pigweed] python_runner: fix target finding at root

At the root level, where the default dummy toolchain is used, the stamp
tool is just 'stamp', so <TARGET_FILE()> wasn't working for them.

Now does.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/47400

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 9c771de68ffba83..71ee193e797b49a
Roller-URL: https://ci.chromium.org/b/8836191062986625969
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8836191062986625969
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I8bbefb0eca2afc4b587bb61f066719d151548a8a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/60689
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 9c771de..71ee193 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 9c771de68ffba834cb94c5eb4dc7edc2cb347f0f
+Subproject commit 71ee193e797b49a11775fa5f8cbfc00673e5fb0e