ffmpeg-full: fix the build phase

I was surprised the build is so slow, and I see it's single-threaded
due to all work being done in install phase :-/
Please, do *not* rewrite phases unless you're really confident it's
a good approach; typically postFoo and preFoo is much better.

I considered filing this ffmpeg-related string of changes as a PR,
but in the end it doesn't seem likely to need review and we have too
many PRs as it is...