fix(ci): remove rustup update, rely on rust-toolchain.toml

[escapedcat]

The CI was broken because rustup update installs latest stable (1.94.1) which hits a recursion limit in matrix-sdk 0.16.0. With rust-toolchain.toml pinning 1.93.0, rustup auto-installs the correct version — no explicit toolchain step needed.

Summary by CodeRabbit

• Chores
  • Streamlined the CI/CD preview workflow to optimize build execution speed by removing unnecessary toolchain update steps.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 65013388-f9cc-4d26-9d79-76df5fb6481b

📥 Commits

Reviewing files that changed from the base of the PR and between 1a5de56 and 3c9e724.

📒 Files selected for processing (1)

• .github/workflows/preview.yml

💤 Files with no reviewable changes (1)

• .github/workflows/preview.yml

---

📝 Walkthrough

Walkthrough

The pull request removes the rustup update step from the GitHub Actions preview workflow, eliminating an explicit toolchain update during CI execution. The workflow now proceeds directly from code checkout to running cargo test.

Changes

Cohort / File(s)	Summary
CI Workflow Configuration .github/workflows/preview.yml	Removed the "Install the latest toolchain" step that executed rustup update, streamlining the preview workflow pipeline.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Possibly related PRs

• docs: add local development setup guide and pin Rust 1.93.0 #77: Complements this change by introducing rust-toolchain.toml to pin the Rust toolchain version, shifting from dynamic updates to explicit version management in CI.

Suggested reviewers

• bubelov

Poem

🐰 One toolchain step, now retired with care,
No more updates floating through the air!
With versions pinned, the builds run steady,
CI pipelines swift and oh, so ready! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: removing the rustup update step from CI and relying on rust-toolchain.toml instead, which directly matches the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/ci-toolchain

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}