Bump third_party/pigweed/repo from 834e876 to 4390d2a

Bumps [third_party/pigweed/repo](https://github.com/google/pigweed) from `834e876` to `4390d2a`.
- [Release notes](https://github.com/google/pigweed/releases)
- [Commits](google/pigweed@834e876...4390d2a)

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

Signed-off-by: dependabot[bot] <[email protected]>