Enable systemd-tmpfiles to create subvols

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

overlay/usr/lib/systemd/system/systemd-tmpfiles-setup.service.d/force-subvol.conf

@@ -0,0 +1,2 @@
+[Service]
+Environment=SYSTEMD_TMPFILES_FORCE_SUBVOL=1