FileNotFoundError

**Hello! I get one problem.**

When I run `manimgl start.py SquareToCircle -o` , the git bash return the error below.

```
Traceback (most recent call last):
File "D:\D\Anaconda\envs\math\Scripts\manimgl-script.py", line 33, in
sys.exit(load_entry_point('manimgl', 'console_scripts', 'manimgl')())
File "d:\d\github\manim\manimlib\__main__.py", line 25, in main
scene.run()
File "d:\d\github\manim\manimlib\scene\scene.py", line 116, in run
self.file_writer.begin()
File "d:\d\github\manim\manimlib\scene\scene_file_writer.py", line 207, in begin
self.open_movie_pipe(self.get_movie_file_path())
File "d:\d\github\manim\manimlib\scene\scene_file_writer.py", line 266, in open_movie_pipe
self.writing_process = sp.Popen(command, stdin=sp.PIPE)
File "D:\D\Anaconda\envs\math\lib\subprocess.py", line 858, in __init__
self._execute_child(args, executable, preexec_fn, close_fds,
File "D:\D\Anaconda\envs\math\lib\subprocess.py", line 1311, in _execute_child
hp, ht, pid, tid = _winapi.CreateProcess(executable, args,
FileNotFoundError: [WinError 2] 系统找不到指定的文件。
```

Can I get some help, please?

當透過 subprocess 嘗試開啟 movie pipe 時，錯誤發生在 scene_file_writer.py 中。檢查傳遞給 Popen 的命令，並確保必要的可執行檔（可能是 ffmpeg）已在 Windows 上安裝並位於 PATH 中。查看 get_movie_file_path 方法和 open_movie_pipe 邏輯以查看缺少哪個檔案。執行一個最小測試，以確認 ffmpeg 是否可從環境存取。

由索引模型根據 Issue 內容生成。