arrow

MINOR: [Archery] Fixed ArrowSources.__str__() not returning string (#35386)



### Rationale for this change

When no git repository is initialised in the `arrow/` directory, archery is supposed to raise an error when executing for example `archery benchmark list`. This error raises another error because the class ArrowSources does not have a proper `__repr__()` method. Below an example traceback:
```sh
Traceback (most recent call last):
  File "/opt/homebrew/bin/archery", line 33, in <module>
    sys.exit(load_entry_point('archery', 'console_scripts', 'archery')())
  File "/opt/homebrew/lib/python3.10/site-packages/click/core.py", line 1130, in __call__
    return self.main(*args, **kwargs)
  File "/opt/homebrew/lib/python3.10/site-packages/click/core.py", line 1055, in main
    rv = self.invoke(ctx)
  File "/opt/homebrew/lib/python3.10/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/opt/homebrew/lib/python3.10/site-packages/click/core.py", line 1657, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/opt/homebrew/lib/python3.10/site-packages/click/core.py", line 1404, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/opt/homebrew/lib/python3.10/site-packages/click/core.py", line 760, in invoke
    return __callback(*args, **kwargs)
  File "/opt/homebrew/lib/python3.10/site-packages/click/decorators.py", line 26, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/Users/philipgroet/projects/afstuderen/arrow/dev/archery/archery/cli.py", line 419, in benchmark_list
    runner_base = CppBenchmarkRunner.from_rev_or_path(
  File "/Users/philipgroet/projects/afstuderen/arrow/dev/archery/archery/benchmark/runner.py", line 222, in from_rev_or_path
    src_rev, _ = src.at_revision(rev_or_path, clone_dir)
  File "/Users/philipgroet/projects/afstuderen/arrow/dev/archery/archery/utils/source.py", line 151, in at_revision
    raise ValueError("{} is not backed by git".format(self))
TypeError: __str__ returned non-string (type PosixPath)
```

### What changes are included in this PR?

The `self.path` attribute of ArrowSources is of type Path, we stringify it.

### Are these changes tested?

Yes, now prints: `ValueError: /Users/philipgroet/projects/afstuderen/arrow is not backed by git`

### Are there any user-facing changes?

No, bugfix

Authored-by: Philip Groet <philip.groet@gmail.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>