roll: third_party/pigweed pw_build: Add pw_rust_crates_extension

This bazel extension allows a downstream project to override the
pigweed rust_crates repo when need be, without requiring every
downstream project to define one.

Fix: 384536812
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/254952
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Original-Revision: ee6f9976ba7e78737a1a74ae05f06da397b896c8

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: ac4e444390b13e..ee6f9976ba7e78
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8728266339523015457
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I7281f060b4b961d85d2663c8110644fb43d6ec57
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/examples/+/255077
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index ac4e444..ee6f997 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit ac4e444390b13e888601520db276b59aad57b4ac
+Subproject commit ee6f9976ba7e78737a1a74ae05f06da397b896c8