flatbuffer builder reuse #4701

Job	Run time
ci (ubuntu-20.04, gcc, g++, 20, Release)	7m 53s
ci (ubuntu-20.04, gcc, g++, 20, Debug)	7m 12s
ci (ubuntu-20.04, clang, clang++, 20, Release)	8m 1s
ci (ubuntu-20.04, clang, clang++, 20, Debug)	6m 59s
ci (ubuntu-22.04, gcc, g++, 20, Release)	7m 22s
ci (ubuntu-22.04, gcc, g++, 20, Debug)	6m 37s
ci (ubuntu-22.04, clang, clang++, 20, Release)	8m 50s
ci (ubuntu-22.04, clang, clang++, 20, Debug)	8m 0s
ci (macos-12, gcc, g++, 20, Release)	7m 56s
ci (macos-12, gcc, g++, 20, Debug)	5m 43s
ci (macos-14, clang, clang++, 20, Release)	4m 6s
ci (macos-14, clang, clang++, 20, Debug)	4m 15s
ci (windows-2022, cl, cl, 20, Release)	7m 15s
ci (windows-2022, cl, cl, 20, Debug)	6m 10s
	1h 36m 19s

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