Make FFmpeg Unix scripts run without sudo

Our FFmpeg scripts for Unix hosts all install into /usr/local, which is
root-owned directory, forcing all parts of these scripts to require
elevated privileges.

This patch makes us write our installed FFmpeg variants into
$HOME/ffmpeg-install/<variant>. We stop using
InstallFromCompressedFileFromURL since the implementation forces all
outputs to be created with root-owner.

Pick-to: 6.12
Change-Id: Id59c23e3160abec61a2e21d1f8f89c4623620b06
Reviewed-by: Assam Boudjelthia <assam.boudjelthia@qt.io>
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>