fix(ci): use prebuilt cargo-nextest binary to avoid rustc version mismatch

cargo-nextest 0.9.130 requires rustc 1.91 but CI pins 1.88.0.
Switch to taiki-e/install-action which downloads a prebuilt binary,
matching the pattern already used for cargo-llvm-cov.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: haasonsaas

TODO.md

@@ -145,7 +145,7 @@ This roadmap is derived from deep research into Greptile's public docs, blog, MC
 94. [x] Add eval fixtures for multi-hop graph reasoning across call chains and contract edges.
 95. [x] Add eval runs that compare single-pass review against agentic loop review.
 96. [ ] Add production replay evals using anonymized accepted/rejected review outcomes.
-97. [ ] Add leaderboard reporting for reviewer usefulness metrics, not just precision/recall.
+97. [x] Add leaderboard reporting for reviewer usefulness metrics, not just precision/recall.
 98. [x] Add regression gates for feedback coverage, verifier health, and lifecycle-state accuracy.
 99. [ ] Add model-routing policies that explicitly separate generation, verification, and auditing roles.
 100. [ ] Publish a repeatable "independent auditor" benchmark story in the UI and CLI so DiffScope's differentiation is measurable.