Mount the existing sccache-cpu PVC and set RUSTC_WRAPPER=sccache for
the regular service-build pipeline. Previously only train-template and
train-multi-seed-template used sccache; the production CI build had
zero rustc-level caching, so every CI run rebuilt every dep from
scratch.
Also set CARGO_INCREMENTAL=0 to override the workspace-wide
[build] incremental = true in .cargo/config.toml. sccache documented
limitation: cannot cache incremental rustc output. Local dev keeps
incremental via config.toml unchanged.
Print sccache --show-stats at end of build for visibility into cache
hit rates.
Mirror of train-template.yaml's sccache pattern; uses the sccache-cpu
PVC (20Gi, scw-bssd-retain) which was already provisioned but unmounted.
Expected impact: rustc cache hit rate jumps from 0% → 50-90% on
subsequent CI runs.
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