[roll third_party/pigweed] CMake build: Change how third party libraries are included

- Always use dir_pw_third_party_nanopb as a path.
- Improve error messages when building Nanopb protos without
  dir_pw_third_party_nanopb set.
- Specify python3 instead of python in case the build isn't run in a
  Python 3 virtual environment.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/38400

third_party/pigweed Rolled-Commits: 7df6664d5739fa1..8f357f439ea576c
Roller-URL: https://ci.chromium.org/b/8852465449253781392
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8852465449253781392
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I47730c25a58d833f56c2eb4898d9f9771bdf2e5e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/38441
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 7df6664..8f357f4 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 7df6664d5739fa101c041ff726c1f0345ec0fbec
+Subproject commit 8f357f439ea576c9d19430e3b6b27b274bf50e95