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>
RHEL-9.4 will be run in parallel with RHEL-9.2 with insignificant tests
until RHEL-9.4 is confirmed stable, then it will replace RHEL-9.2.
[CI Platforms]
Pick-to: 6.8
Task-number: QTQAINFRA-6362
Change-Id: I92c10a15346e7bd0c0190a8d6a6513cf6143d2a8
Reviewed-by: Simo Fält <simo.falt@qt.io>
Reviewed-by: Tero Heikkinen <tero.heikkinen@qt.io>
