Multiple scenes are saved to the same file.

## Description of bug / unexpected behavior

If you have multiple scenes they all get saved to the same file. Very similar to #1578.

### `main.py`

```py
from manim import *

config.background_color = "#333388"
config.frame_width = 40
config.scene_names = ["A", "B"]

class A(Scene):
def construct(self):
a = Circle(5)
self.play(Create(a))

class B(Scene):
def construct(self):
a = Circle(5)
self.play(Create(a))
```

### `manim.cfg`

```
[CLI]
frame_rate = 30
pixel_height = 1080
pixel_width = 1920
```

## Logs
Terminal output

Uhm... `-v DEBUG` does not work:

```
manim -v DEBUG render main.py
Manim Community v0.10.0

Traceback (most recent call last):
File "/usr/local/bin/manim", line 8, in
sys.exit(main())
File "/usr/local/lib/python3.9/site-packages/click/core.py", line 829, in __call__
return self.main(*args, **kwargs)
File "/usr/local/lib/python3.9/site-packages/click/core.py", line 782, in main
rv = self.invoke(ctx)
File "/usr/local/lib/python3.9/site-packages/click/core.py", line 1259, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "/usr/local/lib/python3.9/site-packages/click/core.py", line 1066, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/usr/local/lib/python3.9/site-packages/click/core.py", line 610, in invoke
return callback(*args, **kwargs)
File "/usr/local/lib/python3.9/site-packages/manim/cli/render/commands.py", line 132, in render
for SceneClass in scene_classes_from_file(file):
File "/usr/local/lib/python3.9/site-packages/manim/utils/module_ops.py", line 124, in scene_classes_from_file
module = get_module(file_path)
File "/usr/local/lib/python3.9/site-packages/manim/utils/module_ops.py", line 51, in get_module
raise FileNotFoundError(f"{file_name} not found")
FileNotFoundError: render not found
```

Here's the output without that:

```
manim render main.py
Manim Community v0.10.0

[09/24/21 20:12:03] INFO Animation 0 : Using cached data (hash : 2103384290_1959254436_1145583325) cairo_renderer.py:107
INFO scene_file_writer.py:606
File ready at '/.../project/media/videos/main/1080p60/A.mp4'

INFO Rendered A scene.py:233
Played 1 animations
INFO Animation 0 : Using cached data (hash : 2103384290_950503052_1145583325) cairo_renderer.py:107
INFO scene_file_writer.py:606
File ready at '/.../project/media/videos/main/1080p60/A.mp4'

INFO Rendered B scene.py:233
Played 1 animations

```

## System specifications

System Details

- OS: Mac
- Python version: 3.9.7

Start with the scene discovery flow in manim/cli/render/commands.py and the output-writing path referenced as scene_file_writer.py. Reproduce the issue with the provided main.py and manim.cfg, then compare the output path reported for scenes A and B. Done means separate scene files are produced and the relevant regression test passes.

Written by the indexing model from the issue text.