Fix unininstall if installed to non-default folder

* hptool/os-extras/win/template/Nsisfile.nsi.mu

In either 32- or 64-bit installations, if the user
chooses a non-default install location, a recent
change caused the uninstall to always revert to the
default location.  This change fixes that problem.
Can you add a comment saying why the `isInstall` parameter is needed?