Consider restoring the HasPackageIdentity check on the updater on Windows

A part of the big updater commit is removing these lines:

#ifdef XP_WIN
  // If we're in a package, we know any updates that we find are not for us.
  if (mozilla::widget::WinUtils::HasPackageIdentity()) {
    return NS_OK;
  }
#endif

However, I think that if it isn't unsupported (mingw problems) or harmful (how?) we could restore that check. It might be useful to Mullvad Browser if we eventually create a Windows Store version.