Bump third_party/nanopb/repo from `9766c45` to `839156b` #886

Job	Run time
REPL Tests - Darwin	0s
Test Suites - Darwin (no-ble-asan-clang)	1h 12m 45s
REPL Tests - Linux	41m 24s
Test Suites - Darwin (no-ble-tsan-clang)	1h 46m 6s
Test Suites - Linux (no-ble-tsan-clang)	1h 23m 13s
	5h 3m 28s

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