[roll third_party/pigweed] cmake: Set the PW_ROOT env variable in pigweed.cmake

Instead of setting the PW_ROOT environment variable inside of the
root CMakeLists.txt, it is set if if needed through
pw_build/pigweed.cmake.

In addition, the provided toolchain files are updated to use the
relative path to ../../pw_build/pigweed.cmake to ensure they work
in environments which are not bootstraped.

This ensures that both the upstream toolchains and Pigweed's
CMake tooling in general can be used outside of a bootstrapped
environment.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/74360

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 5825e5c1b1c82d1..648b210c92e6267
Roller-URL: https://ci.chromium.org/b/8827764374912789665
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8827764374912789665
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I918d0971d2a7645f739f520405be6418e532aa22
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/74440
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 5825e5c..648b210 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 5825e5c1b1c82d1563cac8a992d4c5d8f81db422
+Subproject commit 648b210c92e62673262c9a028bbb16d0694925e1