[roll third_party/pigweed] pw_env_setup: Clear PW_ENVIRONMENT_ROOT

Clear PW_ENVIRONMENT_ROOT after bootstrap completes. This reduces the
errors when switching between workspaces in the same terminal.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/127032

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 500dbcca8476653..8d2bb3f418ccf31
Roller-URL: https://ci.chromium.org/b/8790479695092174657
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I52ec740d47f0f71d46af387a8944b2e0dfb55582
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/127077
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 500dbcc..8d2bb3f 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 500dbcca8476653fb8bca18a30e419a50123d3d8
+Subproject commit 8d2bb3f418ccf3106ee4efb90d0a07bca1c04170