(maint) only check builtin_command? for posix

The `builtin_command?` only is useful in posix, where the `type`
function is defined. Windows Powershell does not have `type` defined.

lib/facter/custom_facts/core/execution/base.rb

@@ -138,9 +138,9 @@ def log_stderr(msg, command, logger)
           logger.debug(format(STDERR_MESSAGE, command, msg.strip))
         end
 
+        # optional function to be defined in subclass if necessary
         def builtin_command?(command)
-          output, _status = Open3.capture2("type #{command}")
-          output.chomp =~ /builtin/ ? true : false
+          nil
         end
       end
     end

lib/facter/custom_facts/core/execution/posix.rb

@@ -11,6 +11,11 @@ def search_paths
           ENV['PATH'].split(File::PATH_SEPARATOR) + DEFAULT_SEARCH_PATHS
         end
 
+        def builtin_command?(command)
+          output, _status = Open3.capture2("type #{command}")
+          output.chomp =~ /builtin/ ? true : false
+        end
+
         def which(bin)
           if absolute_path?(bin)
             return bin if File.executable?(bin) && FileTest.file?(bin)