Bump third_party/pigweed/repo from `1f12d06` to `8512b20` #1204

Job	Run time
Build on Darwin (clang, python_lib, simulated)	2m 43s
Build on Linux (fake, gcc_release, clang, simulated)	1h 43m 12s
Build on Linux (coverage)	17m 33s
Build on Linux (gcc_debug)	1h 13m 9s
Build on Linux (python_lib)	1m 56s
Build on Linux (python lighting-app)	2m 47s
	3h 21m 20s

Provide feedback