Prevent windows assistant from trying (and failing) to upgrade itself,
which has never been supported on windows.
The new windows build is made with UPGRADE_LOCATION set, which enabled this
code path that had never run on windows before, and doesn't work. I don't
want to try to support self-upgrade on windows, or generally on other OS's
than the ones where its working, so added a check for that. This way the
build can keep setting UPGRADE_LOCATION and if some later git-annex does
learn how to upgrade itself on some OS, it won't need changing the build
setup.
