ci.yml: the ffmpeg7 PPA fetch is a single point of failure that can hang a PR indefinitely

Observed 2026-08-05 on PR #159: the x86 check job hung ~35 minutes inside 'sudo add-apt-repository -y ppa:ubuntuhandbook1/ffmpeg7' (Launchpad fetch stall) before any toolchain or cargo step, while the arm64 twin passed the same commit. A stuck job cannot be re-run without cancelling first, so a hung PPA fetch blocks a PR indefinitely.

Options: add a timeout to the PPA step, cache/vendor the ffmpeg packages, or fall back to a mirror. Filed under the owner's standing issue authorization to keep track.