Bump third_party/pigweed/repo from 8ffdd15 to 15e7d20

Bumps [third_party/pigweed/repo](https://github.com/google/pigweed) from `8ffdd15` to `15e7d20`.
- [Commits](google/pigweed@8ffdd15...15e7d20)

---
updated-dependencies:
- dependency-name: third_party/pigweed/repo
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

third_party/pigweed/repo

@@ -0,0 +1 @@
+15e7d200f03965658d73a904bb8496398caa79d7