CI: make sccache resilient to transient cache-backend failures #3273

The sccache daemon currently aborts the build when the remote cache backend returns a transient error at startup. Example from the recent GitHub/Azure outage in build-wheel · PR 3239, step 23 L881:

sccache: error: Server startup failed: cache storage failed to read:
Unexpected (temporary) at read =>
<Error><Code>ServerBusy</Code><Message>Egress is over the account limit.

The compile would have succeeded without the cache; the daemon's pre-flight check is what killed the job. We want sccache to degrade gracefully to a cacheless build in this case.

Proposal — try the env vars suggested by @trxcllnt offline

• SCCACHE_SKIP_CACHE_CHECK=true — skip the pre-flight .sccache_check read/write that is the actual thing failing here. Documented in mozilla/sccache docs/Configuration.md; parsed in src/config.rs (skip_cache_check_from_env). Per Paul, cache errors after startup already do not fail the build; the pre-flight is the only hard gate.
• SCCACHE_CACHE_RATE_LIMIT_ON_ERROR_COUNT=5 and SCCACHE_CACHE_RATE_LIMIT_ON_ERROR_WINDOW_SIZE_SECS=5 — temporarily disables cache I/O for 5s after 5 errors in a 5s window. Parsed in the rapidsai fork at src/config/client.rs:730-731 (confirmed present at the v0.18.0-rapids.7 tag that install-sccache pins). Not currently in upstream mozilla/sccache.

Where to apply

• .github/workflows/build-wheel.yml — the failing path uses the GHA backend; this is where the Azure-blob-storage read fired. Add the env vars alongside the existing SCCACHE_GHA_ENABLED=true / SCCACHE_PREPROCESSOR_CACHE_GHA_ENABLED=true block.
• .github/workflows/test-sdist-linux.yml — mirror if the behavior is useful for the local-disk sccache path too.

Related: #1061 (original sccache rollout).

-- Leo's bot