Protobuf Maven Plugin protocDigest is ignored when using protoc from PATH

Summary

### Summary The expected `protocDigest` is ignored when protoc is taken from the `PATH`. ### Details The documentation for the `protocDigest` parameter says: > ... Users may wish to specify this if using a `PATH`-based binary ... However, when specifying `<protoc>PATH</protoc>` the `protocDigest` is not actually checked because the code returns here already https://github.com/ascopes/protobuf-maven-plugin/blob/59097aae8062c461129a13dcda2f4116b90a8765/protobuf-maven-plugin/src/main/java/io/github/ascopes/protobufmavenplugin/protoc/ProtocResolver.java#L91-L93 before the digest check: https://github.com/ascopes/protobuf-maven-plugin/blob/59097aae8062c461129a13dcda2f4116b90a8765/protobuf-maven-plugin/src/main/java/io/github/ascopes/protobufmavenplugin/protoc/ProtocResolver.java#L106 ### PoC Specify: ```xml <protoc>PATH</protoc> <protocDigest>sha256:0000000000000000000000000000000000000000000000000000000000000000</protocDigest> ``` And notice how the `protoc` on the `PATH` is not rejected, despite a digest mismatch. ### Impact Users who have an untrusted `protoc` executable on their `PATH` and rely `<protocDigest>` as protection are affected.