plugin should support system search path

[YifeiZhuang]

This doc is not clear

protobuf-gradle-plugin/README.md

Line 179 in d7c23d9

// If you don't locate it, protoc will try to use "protoc-gen-grpc" from

This configuration generates command --plugin=protoc-gen-grpc=protoc-gen-grpc, and only works when protoc-gen-grpc exists in the current build directory. It does not search the system path.

plugins {
    grpc {

    }
      generateProtoTasks {
        all()*.plugins { grpc {} }
    }
  }

if the entire plugins configuration is omitted, then it generates command without --plugin flag, which searches for system path. Maybe make the above configuration searching for system path is better behavior, because it allows using system path and configuration each plugin separately.

[YifeiZhuang]

related to #699