Fix #1433

[s1as3r]

Fix #1433

Description

Since spotdl uses os.chdir to change the download location, providing
a relative path as an argument to --ffmpeg causes it to look for ffmpeg
in the incorrect location, i.e arguments.ffmpeg realtive to the directory we
change to.

The fix is simple, use absolute path to ffmpeg before we change the working directory.

Related Issue

#1433

How Has This Been Tested?

pytest (python 3.9)

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have read the CORE VALUES document
• I have added tests to cover my changes
• All new and existing tests passed