flatbuffer builder reuse #4721

Job	Run time
ci (ubuntu-20.04, 18, Release)	6m 9s
ci (ubuntu-20.04, 18, Debug)	5m 37s
ci (ubuntu-22.04, 20, Release)	6m 34s
ci (ubuntu-22.04, 20, Debug)	5m 22s
ci (ubuntu-24.04, 20, -Db_sanitize=address, Release)	8m 34s
ci (ubuntu-24.04, 20, -Db_sanitize=address, Debug)	6m 4s
ci (ubuntu-24.04, 20, -Db_sanitize=undefined, Release)	9m 23s
ci (ubuntu-24.04, 20, -Db_sanitize=undefined, Debug)	6m 17s
ci (ubuntu-24.04, 20, -Db_sanitize=thread, Release)	6m 51s
ci (ubuntu-24.04, 20, -Db_sanitize=thread, Debug)	5m 52s
ci (macos-12, 18, Release)	10m 2s
ci (macos-12, 18, Debug)	8m 7s
ci (macos-14, 20, Release)	4m 32s
ci (macos-14, 20, Debug)	4m 46s
ci (windows-2022, 20, Release)	9m 32s
ci (windows-2022, 20, Debug)	7m 32s
	1h 51m 14s

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