Use correct version of butler.exe

[seleb]

Currently, gui-butler comes with a copy of butler.exe, which is not necessarily the latest version. gui-butler should provide a way to call "butler update" in order to remove the need for me to update the build on itch whenever butler updates.

Another possible solution would be to default to the one in the itch app's install location (or on the user's PATH) and use the local copy as a fallback.