[roll third_party/pigweed] pw_watch: Don't search for build dirs; run gn gen if needed

- No longer search for a build.ninja file because sometimes this finds a
  random build.ninja in a presubmit directory. Instead, default to out/
  for the build directory if none is specified.
- Automatically run `gn gen` if a build directory is missing a
  build.ninja file.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/60560

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: bd6dc63d927c0d2..d4a847efe877181
Roller-URL: https://ci.chromium.org/b/8836242996223927601
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8836242996223927601
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Id1f40e04f5edde0461fce11ebf6d5eea03c99598
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/60684
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 bd6dc63..d4a847e 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit bd6dc63d927c0d26583787339226927ed312be0a
+Subproject commit d4a847efe877181891c6e1109b62c118fa9aa81a