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

Job	Run time
Unit / Integration Tests (main)	46m 11s
Unit / Integration Tests (clang)	1h 33m 8s
Unit / Integration Tests (mbedtls)	24m 3s
Unit / Integration Tests (rotating_device_id)	26m 54s
Unit / Integration Tests (icd)	34m 46s
	3h 45m 2s

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