protobuf: allow compiling without `protoc` #3

Job	Run time
Clippy	1m 22s
Check	1m 36s
Rustfmt	27s
Cross compile (wasm32-unknown-unknown)	18s
Cross compile (powerpc64-unknown-linux-gnu)	21s
Cross compile (powerpc-unknown-linux-gnu)	19s
Test Suite	1m 51s
Cross compile (armv7-unknown-linux-gnueabihf)	19s
Check rustdoc intra-doc links	38s
	7m 11s

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