feat(modules/nixos/services/btrbk)!: move '/nix/btrbk' to '/btrbk'

Move the '/nix/btrbk' directory to the '/btrbk' Btrfs subvolume because
Btrbk has nothing to do with '/nix'.

Initially, '/nix/btrbk' was used because everything within '/nix'
already persisted across reboots without any additional efforts.

BREAKING CHANGE: The Btrbk snapshot directory has moved from the
'/nix/btrbk' directory to the '/btrbk' Btrfs subvolume.

To create the new '/btrbk' subvolume, run the following script:

    #!/usr/bin/env bash

    MNTPOINT=$(mktemp -d)
    mount /dev/mapper/luks "$MNTPOINT" -o subvol=/
    trap 'umount $MNTPOINT; rm -rf $MNTPOINT' EXIT
    SUBVOL_ABS_PATH="$MNTPOINT//btrbk"
    mkdir -p "$(dirname "$SUBVOL_ABS_PATH")"
    btrfs subvolume create "$SUBVOL_ABS_PATH"

This script is an extract from the script generated by the following
command:

    nix \
      run \
      github:nix-community/disko \
      -- \
      --dry-run \
      --mode format \
      hosts/masterplan/disko.nix

hosts/bluetop/default.nix

@@ -70,7 +70,7 @@
 
               btrbk = {
                 enable = true;
-                snapshotDir = "/nix/btrbk";
+                snapshotDir = "/btrbk";
               };
 
               openssh.enable = true;

hosts/bluetop/disko.nix

@@ -22,6 +22,11 @@
                 extraArgs = ["-f"];
 
                 subvolumes = {
+                  "/btrbk" = {
+                    mountpoint = "/btrbk";
+                    mountOptions = ["compress=zstd" "noatime"];
+                  };
+
                   "/persistent" = {
                     mountpoint = "/persistent";
                     mountOptions = ["compress=zstd" "noatime"];

hosts/masterplan/default.nix

@@ -71,7 +71,7 @@
 
               btrbk = {
                 enable = true;
-                snapshotDir = "/nix/btrbk";
+                snapshotDir = "/btrbk";
               };
 
               openssh.enable = true;

hosts/masterplan/disko.nix

@@ -22,6 +22,11 @@
                 extraArgs = ["-f"];
 
                 subvolumes = {
+                  "/btrbk" = {
+                    mountpoint = "/btrbk";
+                    mountOptions = ["compress=zstd" "noatime"];
+                  };
+
                   "/persistent" = {
                     mountpoint = "/persistent";
                     mountOptions = ["compress=zstd" "noatime"];

modules/nixos/services/btrbk/default.nix

@@ -8,7 +8,7 @@
 
     snapshotDir = lib.mkOption {
       description = "Path to Btrbk's snapshot directory.";
-      example = "/nix/btrbk";
+      example = "/btrbk";
       type = lib.types.str;
     };
   };
@@ -27,7 +27,5 @@
           subvolume = "/home";
         };
       };
-
-      systemd.tmpfiles.rules = ["d ${cfg.snapshotDir} 700"];
     };
 }