Bump third_party/ot-br-posix/repo from `bd15f71` to `7fd5bd1` #430

Job	Run time
Build on Darwin (clang, simulated)	50m 26s
Build on Linux (fake, gcc_release, clang, simulated)	53m 3s
Build on Linux (coverage)	12m 56s
Build on Linux (gcc_debug)	1h 2m 31s
Build on Linux (python_lib)	24m 20s
Build on Linux (python lighting-app)	6m 31s
	3h 29m 47s

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