Use absolute paths in PATH/LD_LIBRARY_PATH

Otherwise, it's only executable from `/app`:

```
$ heroku run bash
Running `bash` attached to terminal... up, run.8774
~ $ which ffmpeg
vendor/ffmpeg/bin/ffmpeg
~ $ echo $PATH
/usr/local/bin:/usr/bin:/bin:vendor/ffmpeg/bin
~ $ mkdir foo
~ $ cd foo
~/foo $ ffmpeg
bash: ffmpeg: command not found
```