flatpak/platform.bst: Set PYTHONUSERBASE to `/var/data/python`
So that host packages don't leak into sandboxes. This is done also by Freedesktop SDK https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/blob/244d07f553ed39ef217fa4017d89ad06fe431dfb/elements/flatpak-images/platform.bst#L26