Every snapshot is labelled as incomplete

Whenever btrbk runs it throws errors. I've had this issue for a while. I was able to reproduce them with btrbk pulled from master

```
sudo ./btrbk run
WARNING: Target subvolume "/btrfs_pool/_btrbk_snap/root.20231103T1617" exists, but is not a receive target of "/btrfs_pool/_btrbk_snap/root.20231103T1617"
WARNING: Please delete stray subvolumes: "btrbk clean /btrfs_pool/_btrbk_snap"
WARNING: Skipping backup of: /btrfs_pool/_btrbk_snap/root.20231103T1617
WARNING: Target subvolume "/btrfs_pool/_btrbk_snap/home.20231103T1617" exists, but is not a receive target of "/btrfs_pool/_btrbk_snap/home.20231103T1617"
WARNING: Please delete stray subvolumes: "btrbk clean /btrfs_pool/_btrbk_snap"
WARNING: Skipping backup of: /btrfs_pool/_btrbk_snap/home.20231103T1617
--------------------------------------------------------------------------------
Backup Summary (btrbk command line client, version 0.33.0-dev)

Date: Fri Nov 3 16:17:10 2023
Config: /etc/btrbk/btrbk.conf

Legend:
=== up-to-date subvolume (source snapshot)
+++ created subvolume (source snapshot)
--- deleted subvolume
*** received subvolume (non-incremental)
>>> received subvolume (incremental)
--------------------------------------------------------------------------------
/btrfs_pool/root
+++ /btrfs_pool/_btrbk_snap/root.20231103T1617
!!! /btrfs_pool/_btrbk_snap/root.20231103T1617
*** /mnt/drive/btrfs_pool/_btrbk_snap/root.20231103T1617

/btrfs_pool/home
+++ /btrfs_pool/_btrbk_snap/home.20231103T1617
!!! /btrfs_pool/_btrbk_snap/home.20231103T1617
*** /mnt/drive/btrfs_pool/_btrbk_snap/home.20231103T1617

NOTE: Some errors occurred, which may result in missing backups!
Please check warning and error messages above.
Please delete stray subvolumes: "btrbk clean /btrfs_pool/_btrbk_snap"
```

```
sudo ./btrbk clean -n
--------------------------------------------------------------------------------
Cleanup Summary (btrbk command line client, version 0.33.0-dev)

Date: Fri Nov 3 16:47:53 2023
Config: /etc/btrbk/btrbk.conf
Dryrun: YES

Legend:
--- deleted subvolume (incomplete backup)
--------------------------------------------------------------------------------
/btrfs_pool/_btrbk_snap/root.*
--- /btrfs_pool/_btrbk_snap/root.20231103T1617

/mnt/drive/btrfs_pool/_btrbk_snap/root.*

/btrfs_pool/_btrbk_snap/home.*
--- /btrfs_pool/_btrbk_snap/home.20231103T1617

/mnt/drive/btrfs_pool/_btrbk_snap/home.*

NOTE: Dryrun was active, none of the operations above were actually executed!
```

My config:

```
timestamp_format long
transaction_log /var/log/btrbk.log
snapshot_dir _btrbk_snap
snapshot_create always
incremental yes
lockfile /var/lock/btrbk.lock
btrfs_commit_delete yes

volume /btrfs_pool
target /btrfs_pool/_btrbk_snap
target /mnt/drive/btrfs_pool/_btrbk_snap

subvolume root
subvolume home
```