Bump third_party/pigweed/repo from cf4291d to 5b918c6

Bumps [third_party/pigweed/repo](https://github.com/google/pigweed) from `cf4291d` to `5b918c6`.
- [Commits](google/pigweed@cf4291d...5b918c6)

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

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