'Couldn't find ffmpeg or avconv' despite having installed them?

Hello, I think I initially posted this in the wrong place, please let me know if this isn't where this belongs. I'm trying to get started with Manim and am having trouble with the set up.

### Context

I installed ffmpeg via "brew install ffmpeg" in Terminal, and also installed libav with "brew install libav". When I put "ffmpeg" or "avconv" in Terminal, all appears to be fine. ffmpeg is in /usr/local/Cellar.

I initially installed manim via "pip3 install manim". Again, seems to be fine upon check. I also did manimgl. I ran into the issue below so then tried a different route:

```
git clone https://github.com/3b1b/manim.git
cd manim
pip install -e .
manimgl example_scenes.py OpeningManimExample
```
The example seems to work (sort-of; LaTeX seemed buggy).

### Problem

Anyway, I opened up Python using IDLE, and pasted the following 'test' code:

```
from manim import *

class SquareToCircle(Scene):
def construct(self):
circle = Circle()
circle.set_fill(PINK, opacity=0.5)
self.play(Create(circle))
```

**Error**:
```
Warning (from warnings module):
File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/site-packages/pydub/utils.py", line 170
warn("Couldn't find ffmpeg or avconv - defaulting to ffmpeg, but may not work", RuntimeWarning)
RuntimeWarning: Couldn't find ffmpeg or avconv - defaulting to ffmpeg, but may not work
```

### Environment
**OS System**: macOS Catalina 10.15.7
**manim version**: installed here: https://github.com/3b1b/manim.git
**Python version**: 3.9.1

The warning originates from pydub/utils.py line 170. Check how manim or pydub locates ffmpeg/avconv executables on macOS. The user installed via brew, so the path may need to be added to the system PATH or configured in manim. Look at manim's configuration or environment variable setup for media tools. Verify the fix by running the example scene again and ensuring the warning disappears.

Scritto dal modello di indicizzazione a partire dal testo della issue.