[roll third_party/pigweed] pw_build: Add working_directory arg to pw_exec

When executing external programs, allow for setting a working directory
the program will be started with.

Also document pw_exec.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/78480

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 3d3e5381480b5ae..dbb0e562397e9a5
Roller-URL: https://ci.chromium.org/b/8825103519354033633
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8825103519354033633
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I019288a013987e26c1bce37921cfd60b82fd70da
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/78701
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 3d3e538..dbb0e56 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 3d3e5381480b5aed5a4d1b01a20c9d847675b413
+Subproject commit dbb0e562397e9a5085d818f94a620410f4d97f34