ci: Nightly fixes (2026-08-31)

[bosconi]

Based on https://buildkite.com/materialize/nightly/builds/18177 and https://buildkite.com/materialize/nightly/builds/18176

Motivation

bin/unused-deps has failed on every nightly since #18169, plus both v26.40.0 RC nightlies (#18172, #18174), on a single finding:

Found 1 unused dependencies:

  src/clusterd/Cargo.toml
      nix

Description

Remove nix from src/clusterd. #38250 moved the nix::sys::statvfs call out of src/clusterd/src/usage_metrics.rs and into src/metrics/src/usage.rs, which declares nix itself. That left the dependency stranded in the clusterd manifest: at the nightly commit there is no nix reference anywhere under src/clusterd/, so the rustc unused_crate_dependencies lint is correct and no [package.metadata.unused-deps] ignore entry is warranted.

Verification

bin/unused-deps locally, which is the failing check itself:

Finished `dev` profile [unoptimized + debuginfo] target(s) in 5m 28s
All dependencies are used.

plus cargo check -p mz-clusterd --lib --bins, clean. The Cargo.lock change is the corresponding one-line removal from the mz-clusterd dependency list.

Tips for reviewer

The check runs only in the nightly (ci/nightly/pipeline.template.yml), not in tests, which is why #38250 could merge with the stranded dependency. That looks like a deliberate cost decision given it is a full-workspace cargo check, so nothing here changes it.

Unlike most PRs in this series this one carries a single fix; the other failures in those two builds are not mechanical (a Terraform provider constraint, handled separately, an SLT flake, and a Workload Replay benchmark regression).

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