Bump third_party/pigweed/repo from `1f12d06` to `a5a1995` #305

Job	Run time
Build on Darwin (clang, python_lib, simulated)	4m 1s
Build on Linux (fake, gcc_release, clang, simulated)	1h 39m 38s
Build on Linux (coverage)	13m 44s
Build on Linux (gcc_debug)	56m 41s
Build on Linux (python_lib)	3m 27s
Build on Linux (python lighting-app)	3m 10s
	3h 0m 41s

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