Fails to build directory, tar or btrfs subvolumes

[TJ]

All attempts to build non disk-image targets 'directory' 'tar' 'subvolume' (lightweight containers) fail with the same error. The only fix appears to be to upgrade to latest mainline code (mkosi version 9). Example:

root@elloe000:~# mkosi -t directory
DISTRIBUTION:
          Distribution: ubuntu
               Release: focal
                Mirror: http://archive.ubuntu.com/ubuntu

OUTPUT:
         Output Format: directory
                Output: /root/image
       Output Checksum: n/a
      Output Signature: n/a
           Output Bmap: n/a
Output nspawn Settings: n/a
           Incremental: no
             Read-only: no
        FS Compression: no
        XZ Compression: no
            Encryption: no
                Verity: no

PACKAGES:
              Packages: none
         Package Cache: none
           Extra Trees: none
        Skeleton Trees: none
          Build Script: none
         Build Sources: /root
  Source File Transfer: none
       Build Directory: none
        Build Packages: none
    Postinstall Script: none
       Finalize Script: none
  Scripts with network: no
       nspawn Settings: none

HOST CONFIGURATION:
    Extra search paths: none
‣ Detaching namespace...
‣ Detaching namespace complete.
‣ Setting up package cache...
‣ Setting up package cache /root/.mkosi-w5vbxui5 complete.
‣ Setting up temporary workspace.
‣ Temporary workspace in /root/.mkosi-4ayq5hqy is now set up.
‣ Mounting image...
Traceback (most recent call last):
  File "/usr/bin/mkosi", line 4539, in <module>
    main()
  File "/usr/bin/mkosi", line 4535, in main
    run_verb(args)
  File "/usr/bin/mkosi", line 4507, in run_verb
    build_stuff(args)
  File "/usr/bin/mkosi", line 4333, in build_stuff
    raw, tar, root_hash = build_image(args, workspace, do_run_build_script=False, cleanup=True)
  File "/usr/bin/mkosi", line 4147, in build_image
    with mount_image(args, workspace.name, loopdev, encrypted_root, encrypted_home, encrypted_srv):
  File "/usr/lib/python3.8/contextlib.py", line 113, in __enter__
    return next(self.gen)
  File "/usr/bin/mkosi", line 961, in mount_image
    if args.xbootldr_partno is not None:
AttributeError: 'CommandLineArguments' object has no attribute 'xbootldr_partno'
Traceback (most recent call last):
  File "/usr/lib/python3.8/weakref.py", line 642, in _exitfunc
    f()
  File "/usr/lib/python3.8/weakref.py", line 566, in __call__
    return info.func(*info.args, **(info.kwargs or {}))
  File "/usr/lib/python3.8/tempfile.py", line 957, in _cleanup
    cls._rmtree(name)
  File "/usr/lib/python3.8/tempfile.py", line 953, in _rmtree
    _rmtree(name, onerror=onerror)
  File "/usr/lib/python3.8/shutil.py", line 715, in rmtree
    _rmtree_safe_fd(fd, path, onerror)
  File "/usr/lib/python3.8/shutil.py", line 656, in _rmtree_safe_fd
    onerror(os.rmdir, fullname, sys.exc_info())
  File "/usr/lib/python3.8/shutil.py", line 654, in _rmtree_safe_fd
    os.rmdir(entry.name, dir_fd=topfd)
OSError: [Errno 16] Device or resource busy: 'root'