[roll third_party/pigweed] pw_presubmit: Pass rng-seed to gn_fuzz_build

This CL adds a new `modified_env` context manager, and uses it to set
the FUZZTEST_PRNG_SEED that FuzzTest uses for seed material. It also
adds a function to convert `PresubmitContext.rng_seed` to the URL-safe,
base64-encoded value expected by FuzzTest.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/149433

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: f36fcd9b7ee313c..2ed7a41a5b66f12
Roller-URL: https://ci.chromium.org/b/8779631034230584689
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I24fefa83e26cc31c4655e2b30ba8ac03ac1075e3
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/149610
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 f36fcd9..2ed7a41 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit f36fcd9b7ee313c7f74f48cdd443111ecc3b0ca1
+Subproject commit 2ed7a41a5b66f12c5e6d20175186d6bab9420509