Bump third_party/nanopb/repo from `5fd8f04` to `423c03b` #1243

Job	Run time
Build on Darwin (clang, python_lib, simulated)	2h 18m 10s
Build on Linux (fake, gcc_release, clang, simulated)	1h 39m 22s
Build on Linux (coverage)	1h 8m 22s
Build on Linux (gcc_debug)	1h 26m 25s
Build on Linux (python_lib)	1h 41m 55s
Build on Linux (python lighting-app)	18m 18s
	8h 32m 32s

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