ci: make sccache optional in Rust validation

[proerror77]

Change contract

Keep Rust Workspace and Rust HFT Engine Fast Lane validating when the optional sccache action cannot reach GitHub REST APIs.

What changed

• Removed mozilla-actions/sccache-action and its wrapper/env wiring from the two affected CI jobs.
• Kept Swatinem/rust-cache and every existing Cargo validation command.

Acceptance evidence

• git diff --check passed.
• Workflow YAML parsed successfully.
• Static scan confirms no remaining sccache action, wrapper, env flag, or action-output cache-key reference in this workflow.
• Focused GitHub Actions validation is pending on this PR.

Out of scope

• Rust source behavior and fix(alpha): gate live-capable formula missions before persistence #67 alpha-gate logic.
• PLOY/release workflow cache design and compiler-cache performance tuning.

Dependency and merge order

Base: main. Merge this PR before rerunning PR #67, so its Rust CI no longer depends on the failing optional action.

Rollout and rollback

CI-only change. Revert this one commit to restore sccache; validation remains functionally equivalent but builds may run slower.

[coderabbitai]

Caution

Review failed

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/ci-sccache-optional

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