The FFmpeg building flow has become too different on linux and macos.
Decoupling the logic makes it more simple to follow.
The CR also fixes nits and shellcheck warnings in install-ffmpeg.
Pick-to: 6.8 6.9
Change-Id: I28639d28c7210809e52ff18ce612da52f8754285
Reviewed-by: Jøger Hansegård <joger.hansegard@qt.io>
