flatbuffer builder reuse #4699

Job	Run time
ci (ubuntu-20.04, gcc, g++, 20, Release)	7m 50s
ci (ubuntu-20.04, gcc, g++, 20, Debug)	7m 17s
ci (ubuntu-20.04, clang, clang++, 20, Release)	8m 1s
ci (ubuntu-20.04, clang, clang++, 20, Debug)	7m 4s
ci (ubuntu-22.04, gcc, g++, 20, Release)	7m 25s
ci (ubuntu-22.04, gcc, g++, 20, Debug)	6m 21s
ci (ubuntu-22.04, clang, clang++, 20, Release)	8m 54s
ci (ubuntu-22.04, clang, clang++, 20, Debug)	7m 56s
ci (macos-12, gcc, g++, 20, Release)	8m 10s
ci (macos-12, gcc, g++, 20, Debug)	10m 48s
ci (macos-14, clang, clang++, 20, Release)	3m 59s
ci (macos-14, clang, clang++, 20, Debug)	3m 41s
ci (windows-2022, cl, cl, 20, Release)	7m 18s
ci (windows-2022, cl, cl, 20, Debug)	6m 21s
	1h 41m 5s

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