Fix Rubocop warnings for Primitive.object_kind_of? manually

Author: andrykonchin

src/main/ruby/truffleruby/core/posix.rb

@@ -146,7 +146,7 @@ def self.attach_function_eagerly(native_name, argument_types, return_type,
         end
 
         if return_type == :string
-          if result.nil? # must not be Primitive.nil?
+          if result.nil? # rubocop:disable TruffleRuby/ReplaceWithPrimitiveNil
             result = nil
           else
             ptr = Truffle::FFI::Pointer.new(Truffle::Interop.as_pointer(result))