Win32::TaskScheduler#trigger_string doesn't work

Our vendored `Win32::TaskScheduler#trigger_string` method doesn't define the `trigger` variable, so the reference to `trigger` on line 576 will cause the `#trigger` method to be invoked with 0 arguments, and that fails with:

in `trigger': wrong number of arguments (0 for 1) (ArgumentError)

This code path doesn't occur in Puppet, just when using puppet as a library.