Optimize CI workflows: add caching, reduce redundant steps

[devin-ai-integration]

Summary

Adds dependency caching and reduces step overhead across artifact.yaml and unusual.yaml CI workflows. No changes to build logic, test commands, or artifacts produced.

Caching additions:

• Swatinem/rust-cache@v2 added to all jobs that install Rust directly (sdist, macos_aarch64, macos_universal2, windows_amd64, windows_aarch64, unusual). Skipped for container-based jobs (manylinux, musllinux) where caching wouldn't persist.
• cache: 'pip' added to actions/setup-python in sdist, windows_amd64, windows_aarch64, and unusual jobs.

Redundancy removal:

• sdist & unusual: Two separate pip install calls consolidated into one (test/integration deps installed upfront alongside maturin, removing a second pip resolution pass).
• sdist: Four integration test steps → one combined step.
• macos jobs: Three integration test steps each → one step (eliminates repeated source .venv/bin/activate).
• unusual: Three integration test steps → one step.

Toolchain modernization (unusual.yaml):

• actions/setup-python updated from v5 → v6.
• Manual curl https://sh.rustup.rs replaced with dtolnay/rust-toolchain@master (removes need for manual PATH prepend in build step).

Step reordering:

• actions/checkout moved earlier in sdist and unusual jobs so cache actions (rust-cache, setup-python pip cache) can read Cargo.lock and requirements*.txt for cache key computation.

Review & Testing Checklist for Human

• Verify unusual.yaml build step works without explicit PATH="$HOME/.cargo/bin:$PATH" — dtolnay/rust-toolchain should add cargo to GITHUB_PATH automatically, but this is a behavioral change. Trigger the unusual workflow and confirm the maturin build step finds cargo.
• Verify Swatinem/rust-cache works on windows_aarch64 — Rust is installed via manual PowerShell script, and CARGO_HOME is set via GITHUB_ENV (available in subsequent steps). Confirm the cache step finds the cargo registry.
• Verify combined pip install in sdist doesn't cause resolution conflicts — Test deps (pytest, numpy, etc.) are now installed alongside maturin before the build instead of after. Confirm no version conflicts arise.
• Run both workflows end-to-end and compare total job durations against a recent baseline run to confirm caching provides the expected speedup on subsequent runs. First run will be a cache miss (expect similar or slightly slower times); second run should show improvement.

Notes

• Container-based jobs (manylinux, manylinux_cross, musllinux_amd64, musllinux_aarch64) are intentionally unchanged since builds happen inside ephemeral containers where host-level caching doesn't apply.
• Integration tests remain sequential within each combined step (not parallelized with &) since the http test starts a daemon on a fixed port that could conflict with other tests.
• The cache-on-failure: true option on rust-cache ensures partial compilation results are cached even on build failures, improving retry speed.

Link to Devin session: https://app.devin.ai/sessions/d3a95b8122444b47a6baa069678ebb49
Requested by: @lalo458

[devin-ai-integration]

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