Enable sccache preprocessor-cache mode for wheel builds

[ramakrishnap-nv]

Summary

• ci/build_wheel.sh already sources rapids-configure-sccache (shared with ci/build_cpp.sh), so sccache itself was already active for wheel-build-libcuopt's C++ compile. What was missing: SCCACHE_S3_USE_PREPROCESSOR_CACHE_MODE / SCCACHE_S3_PREPROCESSOR_CACHE_KEY_PREFIX.
• Without preprocessor-cache mode, sccache's default "direct mode" keys the cache on the literal compiler invocation (e.g. include paths), which tends to miss across CI runs that check out into a fresh container path each time — exactly wheel-build-libcuopt's situation. Preprocessor-cache mode keys on the preprocessed source content instead, so path differences don't cause cache misses.
• Confirmed this is the established RAPIDS pattern, not a guess: cudf's ci/build_wheel.sh and rmm's ci/build_wheel_cpp.sh both set exactly these two vars in the same place. rapids-configure-sccache (gha-tools) does not set them itself, so it's expected to be opt-in per repo.
• ci/build_wheel.sh is shared across libcuopt/cuopt/cuopt_server/cuopt_sh_client wheel builds; package_name is always passed as the first positional arg by all four callers, so this is safe. It's a no-op for the pure-Python packages (cuopt, cuopt_server, cuopt_sh_client) since sccache is never invoked for them anyway — only libcuopt's C++ compile is affected.

Test plan

• wheel-build-libcuopt CI passes
• Compare sccache --show-adv-stats output (already printed at the end of the script) across two PR runs touching the same C++ source, to confirm cache hit rate improves versus the current baseline

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📝 Walkthrough

Walkthrough

Changes

Wheel build caching

Layer / File(s)	Summary
Configure wheel build cache ci/build_wheel.sh	The wheel build exports sccache S3 preprocessor-cache settings with a package-, architecture-, and CUDA-version-specific key prefix.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to d1a2d

This change enables sccache preprocessor-cache mode for wheel builds, with no actionable merge-blocking risk remaining beyond normal checks and review.

Suggested reviewers: msarahan, jameslamb

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: enabling sccache preprocessor-cache mode for wheel builds.
Description check	✅ Passed	The description directly explains the sccache configuration changes, their purpose, scope, and test plan.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 1…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 1 files.

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch enable-wheel-sccache-preprocessor-cache

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CI Test Summary

✅ All 13 test job(s) passed. (2 skipped)