CI fails on Windows with FileNotFoundError when stting up s3_base fixture

## Describe the bug
After commit 9353fc863d0c99ab0427f83cc5a4f04fcf52f1df, the CI fails on Windows  with FileNotFoundError when stting up s3_base fixture. See: https://app.circleci.com/pipelines/github/huggingface/datasets/8151/workflows/5db8d154-badd-4d3d-b202-ca7a318997a2/jobs/50321

Error summary:
```
ERROR tests/test_arrow_dataset.py::test_dummy_dataset_serialize_s3 - FileNotF...
ERROR tests/test_dataset_dict.py::test_dummy_dataset_serialize_s3 - FileNotFo...
```

Stack trace:
```
______________ ERROR at setup of test_dummy_dataset_serialize_s3 ______________
[gw0] win32 -- Python 3.6.8 C:\tools\miniconda3\python.exe

    @pytest.fixture()
    def s3_base():
        # writable local S3 system
        import shlex
        import subprocess
    
        # Mocked AWS Credentials for moto.
        old_environ = os.environ.copy()
        os.environ.update(S3_FAKE_ENV_VARS)
    
>       proc = subprocess.Popen(shlex.split("moto_server s3 -p %s" % s3_port))

tests\s3_fixtures.py:32: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
C:\tools\miniconda3\lib\subprocess.py:729: in __init__
    restore_signals, start_new_session)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <subprocess.Popen object at 0x0000012BB8A4B908>
args = 'moto_server s3 -p 5555', executable = None, preexec_fn = None
close_fds = True, pass_fds = (), cwd = None, env = None
startupinfo = <subprocess.STARTUPINFO object at 0x0000012BB8177630>
creationflags = 0, shell = False, p2cread = -1, p2cwrite = -1, c2pread = -1
c2pwrite = -1, errread = -1, errwrite = -1, unused_restore_signals = True
unused_start_new_session = False

    def _execute_child(self, args, executable, preexec_fn, close_fds,
                       pass_fds, cwd, env,
                       startupinfo, creationflags, shell,
                       p2cread, p2cwrite,
                       c2pread, c2pwrite,
                       errread, errwrite,
                       unused_restore_signals, unused_start_new_session):
        """Execute program (MS Windows version)"""
    
        assert not pass_fds, "pass_fds not supported on Windows."
    
        if not isinstance(args, str):
            args = list2cmdline(args)
    
        # Process startup details
        if startupinfo is None:
            startupinfo = STARTUPINFO()
        if -1 not in (p2cread, c2pwrite, errwrite):
            startupinfo.dwFlags |= _winapi.STARTF_USESTDHANDLES
            startupinfo.hStdInput = p2cread
            startupinfo.hStdOutput = c2pwrite
            startupinfo.hStdError = errwrite
    
        if shell:
            startupinfo.dwFlags |= _winapi.STARTF_USESHOWWINDOW
            startupinfo.wShowWindow = _winapi.SW_HIDE
            comspec = os.environ.get("COMSPEC", "cmd.exe")
            args = '{} /c "{}"'.format (comspec, args)
    
        # Start the process
        try:
            hp, ht, pid, tid = _winapi.CreateProcess(executable, args,
                                     # no special security
                                     None, None,
                                     int(not close_fds),
                                     creationflags,
                                     env,
                                     os.fspath(cwd) if cwd is not None else None,
>                                    startupinfo)
E                                    FileNotFoundError: [WinError 2] The system cannot find the file specified

C:\tools\miniconda3\lib\subprocess.py:1017: FileNotFoundError
```

Labels: name: bug