Bump prost-build from 0.12.4 to 0.12.6

Bumps [prost-build](https://github.com/tokio-rs/prost) from 0.12.4 to 0.12.6.
- [Release notes](https://github.com/tokio-rs/prost/releases)
- [Commits](tokio-rs/prost@v0.12.4...v0.12.6)

---
updated-dependencies:
- dependency-name: prost-build
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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