ci: cache Rust compilation with sccache

[pjb157]

Summary

• route the four sharded dwctl coverage builds through sccache on Depot runners
• use Depot's runner-provided WebDAV cache credentials, with compiler cache failures treated as non-fatal
• retain Swatinem/rust-cache for Cargo dependencies/tools while disabling its target cache to avoid caching build outputs twice
• pin the sccache action and binary versions
• add a workflow contract test that keeps the cache configuration scoped to the intended job and rejects long-lived/manual cache credentials

Result: not recommended for merge

This reproduces the Depot article's configuration successfully, but it does not improve this repository's existing sharded coverage build. Even with a 100% sccache hit rate, fetching hundreds of individual compiler objects is slower here than restoring the existing compressed Rust target cache and compiling the remaining work locally on a 16-core Depot runner.

Scenario	Median Cargo compile	Median compile + test step	Median shard job	sccache hit rate
Existing workflow baseline	131s	154.5s	211.5s	n/a
Empty sccache	211.5s	239s	368s	4.9–25.1%
Warm sccache, attempt 1	141s	165s	219s	100%
Warm sccache, attempt 2	141s	165s	218s	100%

Against the existing workflow, the two warm attempts consistently regressed median Cargo compilation by 7.6%, the measured compile/test step by 6.8%, and the complete shard job by about 3.1%. The empty-cache attempt was substantially slower, as expected.

The first warm attempt had one failure in test_flush_emits_single_notification_for_multiple_depletions; the second identical warm attempt passed the complete workflow. The timing regression was the same in both attempts, so it is independent of that test flake.

Runs

• Existing workflow baseline
• sccache benchmark: cold plus two warm attempts

Validation

• bash .github/scripts/test-rust-ci-matrix.sh
• ruby scripts/tests/preview_workflow_test.rb (4 runs, 63 assertions)
• shellcheck .github/scripts/test-rust-ci-matrix.sh
• actionlint -ignore 'label "depot-' -ignore 'shellcheck reported issue' .github/workflows/ci.yaml
• complete GitHub Actions workflow passed on the final warm attempt

This PR is intentionally left as a draft experiment. Based on the measurements above, I recommend closing it rather than merging the implementation.

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