Bump third_party/openthread/repo from `266a97b` to `666a9bd` #1229

Job	Run time
Build on Linux (gcc_debug)	46m 35s
Build on Darwin (clang, python_lib, simulated)	1h 41m 49s
Build on Linux (fake, gcc_release, clang, simulated)	1h 23m 37s
Build on Linux (coverage)	10m 44s
Build on Linux (python_lib)	19m 47s
Build on Linux (python lighting-app)	6m 8s
	4h 28m 40s

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