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:
```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.