Protobuf Maven Plugin protocDigest is ignored when using protoc from PATH

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:

<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.