fix(store): propagate cpu time from derivation trampoline

[CertainLach]

Motivation

Right daemon doesn't report CPU time after build, build result has empty stats

Context

Summary by CodeRabbit

• Bug Fixes
  • Improved build result reporting by accurately aggregating completion times, CPU usage, and build counts across related build steps.
  • Build results now provide more consistent timing and resource-use information for composite builds.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: c882d109-3804-447c-8dda-5c843c2907cc

📥 Commits

Reviewing files that changed from the base of the PR and between 31bd4f6 and 44d8c06.

📒 Files selected for processing (1)

• src/libstore/build/derivation-trampoline-goal.cc

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📝 Walkthrough

Walkthrough

This change adds aggregation logic to DerivationTrampolineGoal::haveDerivation that combines timing and CPU usage statistics from individual subgoals into the parent build result, including build counts, start/stop times, and CPU user/system time.

Changes

Build metrics aggregation

Layer / File(s)	Summary
Aggregate subgoal metrics into buildResult src/libstore/build/derivation-trampoline-goal.cc	Adds <chrono> include and a loop over concreteDrvGoals that sums timesBuilt, computes earliest non-zero startTime, takes max stopTime, and sums cpuUser/cpuSystem into the parent buildResult.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the main change: propagating CPU time from the derivation trampoline into build results.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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