btrbk doesn't append a command with "sudo -n" if ssh is used and backend_local_user=btrfs-progs-sudo

I added the commands btrfs, readlink, and test to the sudoers file on both systems to make btrbk without sudo possible. My `btrbk.conf` file is:
```
backend_local_user btrfs-progs-sudo
ssh_user anna

volume ssh://154.186.17.112:22/
snapshot_dir home/user/.snapshots
target /mnt/backup
subvolume home/user
```

When I run `btrbk run -v -progress --dry-run` I get the following warnings:

```
WARNING: Skipping subvolume "154.186.17.112[22]:/home/user": Failed to fetch subvolume detail
WARNING: ... Command execution failed (exitcode=1)
WARNING: ... sh: ssh -p 22 anna@154.186.17.112 'btrfs subvolume list -a -c -u -q -R '\''/home/user'\'''
WARNING: ... can't perform the search: Operation not permitted
```

The warning states that `ssh -p 22 anna@154.186.17.112 'btrfs subvolume list -a -c -u -q -R '\''/home/user'\'''` is the command the program is trying to execute. The btrfs command is not appended with `sudo -n` like the documentation states (`backend_local_user=btrfs-progs-sudo`). `ssh -p 22 anna@154.186.17.112 'sudo -n btrfs subvolume list -a -c -u -q -R '\''/home/user'\'''` would run just fine (with `sudo -n` added). This seems like a bug to me.

If I change ssh_user to root, the program will run fine too, but my goal is to disable root logins for ssh. Is there any way to achieve this?

Start with the backend_local_user and ssh_user handling described in btrbk.conf, then reproduce the issue with `btrbk run -v -progress --dry-run`. Compare the remote command for btrfs subvolume listing with the documented `btrfs-progs-sudo` behavior; done means the remote command includes `sudo -n` and works for a non-root SSH user.

Written by the indexing model from the issue text.