ci: cache sccache local disk via actions/cache for hosted runners

[staging-devin-ai-integration]

Summary

Cache sccache's local disk directory (~/.cache/sccache) as a single GitHub Actions cache entry, giving hosted runners compilation-level cache hits without the per-artifact entry explosion caused by SCCACHE_GHA_ENABLED (reverted in #373).

How it works:

• actions/cache/restore runs before sccache-action — pre-populates the local sccache directory from a prior main-branch run
• sccache finds cached compilation artifacts on disk → cache hits for unchanged crates
• actions/cache/save runs at end of job, main-branch only (if: always() && github.ref == 'refs/heads/main')
• Self-hosted GPU jobs are unchanged — they already have persistent disk sccache

Cache key strategy:

• Main workspace jobs: sccache-{os}-{Cargo.lock hash} with restore-keys: sccache-{os}- for partial matches
• Plugin jobs: sccache-plugins-{os}-{plugins/**/Cargo.lock hash} to avoid key collision

This creates 1 cache entry (vs ~1000 with SCCACHE_GHA_ENABLED) and complements Swatinem/rust-cache — rust-cache handles the target/ directory (linking artifacts, incremental state), sccache handles individual compilation output.

Closes #374

Review & Testing Checklist for Human

• Verify the first CI run on this PR shows Cache not found for sccache (expected — no main-branch entry yet) and sccache stats show 0% hit rate
• After merging to main, verify the main-branch CI run saves the sccache cache entry (check the "Save sccache cache" step)
• On the next main-branch or PR run, verify sccache stats show >0% hit rate (the Sccache stats post-step from mozilla-actions/sccache-action)
• Check https://github.com/streamer45/streamkit/actions/caches — should see 1-2 sccache-Linux-* entries (not hundreds)

Notes

• Release and marketplace workflows save sccache cache unconditionally (no github.ref check) since they only run on tags/dispatch respectively
• Expected sccache cache size: ~1-3 GiB compressed, fitting comfortably within the 10 GiB repo quota alongside the 3 rust-cache entries

Link to Devin session: https://staging.itsdev.in/sessions/cdf841e834eb419aa04168a565311f10
Requested by: @streamer45

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✅ Reviewed on 7a21e12

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✅ Devin Review: No Issues Found

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