`GenerateProtoTask` does not consider path-based `protoc` or plugins as task inputs

[clayburn]

If you set protoc by path rather than by artifact, the GenerateProtoTask does not consider that to be a task input. For example this buildscript:

val protocPath: String? = System.getenv("PROTOC_PATH")
protobuf {
    protoc {
        path = protocPath
    }
}

results in the following:

> PROTOC_PATH=proto-3.19.3/bin/protoc ./gradlew generateProto
> Task :proj:generateProto

> PROTOC_PATH=proto-3.20.1/bin/protoc ./gradlew generateProto
> Task :proj:generateProto UP-TO-DATE

The same also holds true for plugins defined by a path instead of an artifact specification.

[ejona86]

Fixed by #560