compileProtoPath configuration variants are still too vague

The Usage attribute was added in https://github.com/google/protobuf-gradle-plugin/commit/44309a20736dbfa5de3bb3e50a2566fb994ab936
but this configuration is still lacking the category attribute.
This can cause variant selection issues, as it is fairly common for a publication to have multiple variants with the same LibraryElements + Usage.
I suggest specifying the category of "library", unless there is a reason that doesn't work.