[roll pigweed] pw_toolchain: Always set pw_toolchain_CLANG_PREFIX

Sets pw_toolchain_CLANG_PREFIX to directly point to the clang/gcc
compilers pulled down by CIPD rather than assuming the correct binaries
are on the system PATH.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/106750

https://pigweed.googlesource.com/pigweed/pigweed
pigweed Rolled-Commits: 17883430a890cbc..64fb8479feb4af3
Roller-URL: https://ci.chromium.org/b/8805663375690489121
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I9e0a5d7bc49ca3b5aff9fb09227d267c1d6536cb
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/106678
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/pigweed b/pigweed
index 1788343..64fb847 160000
--- a/pigweed
+++ b/pigweed
@@ -1 +1 @@
-Subproject commit 17883430a890cbc6b936bc4b8b98d939680d07d8
+Subproject commit 64fb8479feb4af350f9ff2bea3318c9866ea9c40