homebridge-camera-ffmpeg installation overwrites ffmpeg
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**Describe The Bug:**
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When installing homebridge-camera-ffmpeg on a Rapberry Pi with a self-compiled ffmpeg in /usr/local/bin it will overwrite that ffmpeg version.

**To Reproduce:**
<!-- Steps to reproduce the behavior. -->
- Self-compile and install ffmpeg with prefix /usr/local
- Install homebridge-camera-ffmpeg through config-ui-x

**Expected behavior:**
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The installer should notice theres a version of ffmpeg already installed in /usr/local and refrain from overwriting it

**Logs:**
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```
Show the Homebridge logs here.
Remove any sensitive information.
```

**Homebridge Config:**

```json
Show your homebridge config.json here.
Remove any sensitive information, such as your homebridge-gsh / google-smarthome token.
```

**Screenshots:**
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**Environment:**

* **Node.js Version**: <!-- node -v -->
v10.15.3

* **NPM Version**: <!-- npm -v -->
6.14.4

* **Homebridge Version**: <!-- homebridge -V -->
0.4.53

* **Homebridge Camera FFmpeg Version**:
0.1.17

* **Homebridge Config UI X Plugin Version**:
4.15.1

* **Operating System**: <!-- Raspbian / Ubuntu / Debian / Windows / macOS / Docker -->
Raspbian GNU/Linux 9 (stretch)

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