feat: Impermanence (system)

I had to disable initrd's systemd stuff.  I just /couldn't/ get a rollback
service working; believe me, not for a lack of effort!  I've been working on
this on-and-off for a month or two now.

hosts/nixos-testbed/disko-config.nix

@@ -6,6 +6,9 @@
   boot.initrd.supportedFilesystems.btrfs = true;
   boot.supportedFilesystems.btrfs = true;
 
+  # From Impermanence's README: "Important note: Make sure your persistent
+  # volumes are marked with neededForBoot, otherwise you will run into
+  # problems."
   fileSystems."/persist".neededForBoot = true;
 
   disko.devices = {