build(deps): pin protobuf to 3.20.3 in fuzzer_runner for OSS-Fuzz compatibility #1313

Job	Run time
test (program-model, buttercup.program_model, 3.12)	1m 15s
test (seed-gen, buttercup.seed_gen, 3.12)	1m 17s
test (common, buttercup.common, 3.12)	6m 38s
test (orchestrator, buttercup.orchestrator, 3.12)	1m 8s
test (fuzzer, buttercup.fuzzer, 3.12)	1m 5s
test (fuzzer_runner, buttercup.fuzzer_runner, 3.12)	1m 3s
test (patcher, buttercup.patcher, 3.12)	1m 21s
coverage-upload	7s
	13m 54s

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