Bump third_party/nanopb/repo from `9766c45` to `839156b` #1074

Job	Run time
Build on Darwin (clang, python_lib, simulated)	1h 19m 24s
Build on Linux (fake, gcc_release, clang, simulated)	2h 46m 25s
Build on Linux (coverage)	17m 9s
Build on Linux (gcc_debug)	42m 9s
Build on Linux (python_lib)	18m 34s
Build on Linux (python lighting-app)	5m 45s
	5h 29m 26s

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