[third_party/pigweed] Roll 2 commits

f6c3a78788e7cb8 pw_sync: add Borrowable helper for transactional g
88b0f9b0332f07b pw_env_setup: Include Sphinx themes in bootstrap

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 0ab1d3c64d5e6c6..f6c3a78788e7cb8
Roller-URL: https://ci.chromium.org/b/8836530838514152577
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8836530838514152577
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I6015f417c49e3637e365f616b30cdc44464215a0
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/60343
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 0ab1d3c..f6c3a78 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 0ab1d3c64d5e6c62877632aa661b5fd0052678cd
+Subproject commit f6c3a78788e7cb8fe094f4042e9e26a7559610bc