Bump third_party/pigweed/repo from `cbbc73d` to `9640cde` (#31414)

Bumps [third_party/pigweed/repo](https://github.com/google/pigweed) from `cbbc73d` to `9640cde`.
- [Commits](https://github.com/google/pigweed/compare/cbbc73dc4d56bc201e9d50c4b10db974aff82754...9640cdef100f87d7987875d3a418931d6500e5b2)

---
updated-dependencies:
- dependency-name: third_party/pigweed/repo
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
diff --git a/third_party/pigweed/repo b/third_party/pigweed/repo
index cbbc73d..9640cde 160000
--- a/third_party/pigweed/repo
+++ b/third_party/pigweed/repo
@@ -1 +1 @@
-Subproject commit cbbc73dc4d56bc201e9d50c4b10db974aff82754
+Subproject commit 9640cdef100f87d7987875d3a418931d6500e5b2