Currently `objectstore.Object.{read, write}` directly return
strings but in the future they might return an Object that is
an `os.PathLike`, i.e. has a `__fspath__` method, instead.
Prepare for that by ensuring all `tree`s are converted to their
file system representation via `os.fspath` when needed, e.g.
when creating the bind-mount arguments for the `BuildRoot`.