produce mp3 samples on the CI builds

[atsushieno]

Since we are the only one who can do it, why not!?

• [x] we need audio plugins installed at certain location. Maybe studiorack helps it?
• [x] we need augene-player run headlessly, to achieve (1) audio plugin lookup and cache as JUCE config settings, (2) "export" plugin helper MMLs as part of config files, and (3) "render" tracktionedit files into wav.
• [ ] extend #include processor to make it to load files from the config directory too.
• [x] ffmpeg or whatever to (a) remove silence tail, and (b) convert to mp3 and/or flac.

[atsushieno]

A production pipeline is achieved at atsushieno/augene-ng-production repository, which had a successful MP3 production.

No special care about #include yet, but we can close it.