By default, `systemd-tmpfiles` will create normal directories instead of
Btrfs subvolumes unless `/` is already a subvolume. According to
[Lennart][0], this has to do with subvolumes being too "heavy-weight,"
whatever that means.
Fortunately, we can override this nonsense with an environment variable.
[0]: https://github.com/systemd/systemd/pull/1915
