build(deps): bump buildbot-nix from 6d7abb5 to c989cad

Bumps [buildbot-nix](https://github.com/nix-community/buildbot-nix) from `6d7abb5` to `c989cad`.
- [Release notes](https://github.com/nix-community/buildbot-nix/releases)
- [Commits](nix-community/buildbot-nix@6d7abb5...c989cad)

---
updated-dependencies:
- dependency-name: buildbot-nix
  dependency-version: c989cad7be89494baccfc18ef6c250f8e38b2e8c
  dependency-type: direct:production
...

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Author: dependabot[bot]