Record index build time in buildInMemory path

[r-devulap]

buildInMemory (the no-build-compressor path) never recorded the index build time into indexBuildTimes, so getIndexBuildTimeSeconds() always returned null for it. This caused construction.index_build_time_s to be omitted from benchmark results, triggering a warning whenever the metric was selected in the logging config:

WARNING: selected logging output not available; skipping metrics.construction.index_build_time_s (construction.index_build_time_s)

Fix: capture the duration of builder.build() in buildInMemory and store it in indexBuildTimes, consistent with how buildOnDisk already does it. Write time is intentionally excluded — only graph construction is measured.

Before:

Index build time: null seconds

After:

Index build time: 32.192336 seconds

[github-actions]

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