Bump third_party/pigweed/repo from `1f12d06` to `7940ef2` #2416

Job	Run time
Build on Darwin (clang, python_lib, simulated)	3m 26s
Build on Linux (fake, gcc_release, clang, simulated)	2m 4s
Build on Linux (gcc_debug)	2m 9s
Build on Linux (coverage)	2m 55s
Build on Linux (python_lib)	2m 54s
Build on Linux (python lighting-app)	1m 45s
	15m 13s

Provide feedback