[fugitive] Use btrfs subvolume as root-fs

Using a smaller server right now with less ram to spare

lib/systems/fugitive/hardware.nix

@@ -11,11 +11,44 @@
   boot.kernelModules = [ ];
   boot.extraModulePackages = [ ];
 
+  boot.initrd.enable = true;
+  boot.initrd.supportedFilesystems = [ "btrfs" ];
+  boot.initrd.systemd.enable = true;
+  boot.initrd.systemd.services.rollback = {
+    description = "Rollback BTRFS root subvolume to the blank state";
+    wantedBy = [ "initrd.target" ];
+    after = [ "initrd-root-device.target" ];
+    before = [ "sysroot.mount" ];
+    unitConfig.DefaultDependencies = "no";
+    serviceConfig.Type = "oneshot";
+    script = ''
+    mkdir -p "/mnt"
+    mount -o subvol="/" /dev/disk/by-label/Nixos /mnt
+    
+    echo "Cleanup /root-last"
+
+    # Delete root-last
+    btrfs subvolume list -o /mnt/root-last |
+      cut -f9 -d' ' |
+      while read subvolume; do
+        btrfs subvolume delete "/mnt/$subvolume"
+      done &&
+      btrfs subvolume delete /mnt/root-last
+
+    # Move root to root-last
+    echo "Moving /root subvolume to /root-last"
+    mv /mnt/root /mnt/root-last
+
+    # Restore root subvolume
+    btrfs subvolume snapshot /mnt/root-blank /mnt/root
+    '';
+  };
+
   fileSystems."/" =
     {
-      device = "none";
+      device = "/dev/disk/by-label/Nixos";
-      fsType = "tmpfs";
+      fsType = "btrfs";
-      options = [ "size=2G" "mode=755" ];
+      options = [ "subvol=root" "compress=zstd" ];
     };
 
   fileSystems."/boot" =