build(deps): bump buildbot-nix from d193d37 to 9c6f6b4

Bumps [buildbot-nix](https://github.com/nix-community/buildbot-nix) from `d193d37` to `9c6f6b4`.
- [Release notes](https://github.com/nix-community/buildbot-nix/releases)
- [Commits](nix-community/buildbot-nix@d193d37...9c6f6b4)

---
updated-dependencies:
- dependency-name: buildbot-nix
  dependency-version: 9c6f6b407a77ff39c5c5788e66d04f2bb40628ea
  dependency-type: direct:production
...

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Author: dependabot[bot]