After making `ffmpeg` point to the latest `ffmpeg_4`, all packages that
used `ffmpeg` without requiring a specific version now use ffmpeg_3
explicitly so they shouldn't change.
Uses the HTTPS url for cases where the existing URL has a permanent
redirect. For each domain, at least one fixed derivation URL was
downloaded to test the domain is properly serving downloads.
Also fixes jbake source URL, which was broken.
