Bump third_party/pigweed/repo from `d5fcc90` to `6ad0bec` #339

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 54m 10s
Build on Linux (fake, gcc_release, clang, simulated)	1h 58m 50s
Build on Linux (coverage)	13m 38s
Build on Linux (gcc_debug)	59m 3s
Build on Linux (python lighting-app)	5m 50s
Build on Linux (python_lib)	21m 44s
	5h 33m 15s

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