Fix non-default feature build: Feature gate unpack_ffmpeg with download_ffmpeg

[Wumpf]

Currently compilation fails when building for non-linux without the download_ffmpeg feature.

This happens because the zip library is (correctly) feature gated to download_ffmpeg. This in turn is used by unpack_ffmpeg which wasn't behind the feature flag!

[Wumpf]

Don't know where else to write that: Awesome project! So much nicer than interfacing with the ffmpeg libraries directly Using it in the Rerun visualization tool now :)

[nathanbabcock]

Great catch, thank you for fixing this.

I'm going to merge this and then create a small follow-up to also add the cargo check step for Mac M1, and perhaps also share some of those common steps across jobs.

Thank you for the kind words, and I'm excited to check out Rerun more deeply. If you run across any pain points or ideas for improvements in the future I would love to hear them!