Remove the `{ffmpeg}` variable from the default configuration, and its use
from the job compiler scripts. Now that the Worker can find its bundled
FFmpeg, it's no longer needed to configure its location on the Manager.
Refactor the JS script file loading code so that it's tied to the `fs.FS`
interface for longer, and less to the specifics of our `embed.FS` instance.
This should make it possible to use other filesystems, like a real on-disk
one, to load scripts.
