ci: Fix ffmpeg install failure for Linux+Firefox (#3944)
ci: Fix ffmpeg install failure for Linux+Firefox (#3944)

CI for Firefox on Linux is failing because of (I suspect) an outdated
package index in Ubuntu.  This addresses it by updating the package
list before installing ffmpeg.
tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player