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Bump third_party/ot-br-posix/repo from `bd15f71` to `b574b6c` #3514

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Run time

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Job	Run time
Chef - ESP32 CI Examples	13m 51s
Chef - Linux CI Examples	5m 27s
Chef - NRFConnect CI Examples	7m 50s
Chef - Telink CI Examples	11m 56s
	39m 4s