Change Audiobookshelf mountpoint to service's default

This is being done as a step before moving to the nix version of ABS.
That change would be included here too, but I ran into a bug:

```
[2024-12-02 19:53:46.433] FATAL: [Server] Unhandled rejection: [Error: EACCES: permission denied, mkdir '/metadata'] {
  errno: -13,
  code: 'EACCES',
  syscall: 'mkdir',
  path: '/metadata'
}
promise: Promise {
  <rejected> [Error: EACCES: permission denied, mkdir '/metadata'] {
    errno: -13,
    code: 'EACCES',
    syscall: 'mkdir',
    path: '/metadata'
  }
}

```

No idea why yet so the container lives on.

To setup the mountpoint, I ran `zfs set mountpoint=legacy
orico/audiobookshelf` and created the directory used as the mount in the
`fileSystems` section.

modules/hosts/nixos/nixnuc/containers/audiobookshelf.nix

@@ -1,5 +1,5 @@
 { ... }: let
-  volume_base = "/orico/audiobookshelf";
+  volume_base = "/var/lib/audiobookshelf";
   http_port = "13378";
 in {
   # Audiobookshelf