[roll third_party/pigweed] pw_env_setup: Detect unsupported platforms

Detect OS/arch strings like "linux-amd64" that aren't supported by the
core CIPD executable and skip CIPD setup in that case, assuming the user
has already installed the correct versions of tools.

Example output when "linux-amd64" was removed from the list of supported
platforms. (I do not have the right versions of all the tools installed
on my system.)

Downloading and installing packages into local source directory:

  Setting up CIPD package manager...skipped
    UnsupportedPlatform('linux-amd64',)
    abandoning CIPD setup
  Setting up Python environment.....[|]
gn.py: Could not find checkout in any parent of the current path.
...

Original-Fixed: 349
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/39140

third_party/pigweed Rolled-Commits: 5c26ecfdad33664..a1cb66309e3542c
Roller-URL: https://ci.chromium.org/b/8851942903555880576
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8851942903555880576
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ibabb4798405b1ef1bb7b22ed3cc1f883feb202a8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/39162
Reviewed-by: 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 5c26ecf..a1cb663 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 5c26ecfdad33664689e128e7af498cd91a8612fe
+Subproject commit a1cb66309e3542c02f9e428fe35c49c83440e2a9