ci: enable sccache and disable incremental in rust test job

[jwalin-shah]

Summary

Two stacked wins on the Rust test job (.github/workflows/test.yml):

• CARGO_INCREMENTAL=0 — incremental compilation is for warm local dev, pointless on fresh CI runners. Saves disk + a few % wall-clock.
• RUSTC_WRAPPER=sccache via mozilla-actions/sccache-action@v0.0.9 with SCCACHE_GHA_ENABLED=true. Layers cleanly on top of the existing Swatinem/rust-cache@v2: rust-cache warms target/ per branch; sccache caches individual compilation units across branches. Expected 30–60% off cold Rust builds on fresh PRs.

No changes to build-windows.yml (triggers only on workflow_dispatch / fix/windows branch) or release workflows — those can follow if this proves clean.

Test plan

• First CI run on this PR: sccache step installs cleanly inside the openhuman_ci:rust-1.93.0 container
• Subsequent runs: confirm sccache cache hits in the job log (sccache --show-stats appears at job end from the action)
• No regression in Rust test runtime (Swatinem + sccache should layer, not fight)

If sccache-in-container turns out flaky, the fallback is to revert the sccache step and keep only the CARGO_INCREMENTAL=0 line.

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Run ID: 8a0d5492-9016-4b61-bd7f-80d233da218f

📥 Commits

Reviewing files that changed from the base of the PR and between c0f6e39 and 1fb2901.

📒 Files selected for processing (1)

• .github/workflows/test.yml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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