Simplified build of 220.video-processing

This benchmark relies on a static build of ffmpeg, which we source from an external website. Instead of pulling the ffmpeg binary at every single build, which adds a lot of pressure on that website in CI builds and can lead to errors when pulling data (e.g., rate limits), we could put a pinned and reproducible variant of ffmpeg in our benchmarks-data instead.

We should a new resources category in benchmarks-data, and change init.sh script to copy that binary during the initial build.

EDIT: we should also revise the command in the function to make sure that we explicitly select codecs and are reproducible.

Start by inspecting init.sh and the existing ffmpeg download path, then locate the benchmark function whose command needs explicit codec selection. Add the pinned binary under the new benchmarks-data resources category, update the initial build to copy it, and verify that repeated builds avoid external downloads and use reproducible codecs.