[roll third_party/pigweed] pw_presubmit: Add shellcheck

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/97460

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: cf60b1cddba0b89..6aa72eddd8c9f14
Roller-URL: https://ci.chromium.org/b/8811074035450723361
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8811074035450723361
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I3b62ed372271ad92ffa37c343021978887d4daf5
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/98589
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 cf60b1c..6aa72ed 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit cf60b1cddba0b89b320e1f8dbff5e92a0525118b
+Subproject commit 6aa72eddd8c9f146dfc553c574255b5f8df55f86