Coin, FFmpeg, macOS: Add output parameter to 'install-ffmpeg-macos.sh'

This patch adds an optional parameter to the provisioning script
'install-ffmpeg-macos.sh', allowing us to override the output directory
for which the finalized library is installed.

This is particularly useful for local builds, where we often need
multiple different versions of FFmpeg installed, for different Qt
versions.

This patch has no impact for existing coin workflows.

Pick-to: 6.8
Change-Id: If31922e931db19ab189850b975ce0131a64321b4
Reviewed-by: Simo Fält <simo.falt@qt.io>
(cherry picked from commit 708548742b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 298d280d64)

coin/provisioning/common/macos/install-ffmpeg-macos.sh

@@ -3,6 +3,7 @@
 # SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
 
 # This script builds and installs FFmpeg shared libs
+# Can take an optional final parameter to control installation directory
 
 set -ex
 
@@ -18,7 +19,8 @@ source "${BASH_SOURCE%/*}/../unix/ffmpeg-installation-utils.sh"
 ffmpeg_source_dir=$(download_ffmpeg)
 ffmpeg_name=$(basename "$ffmpeg_source_dir")
 ffmpeg_config_options=$(get_ffmpeg_config_options "shared")
-prefix="/usr/local/$ffmpeg_name"
+default_prefix="/usr/local/$ffmpeg_name"
+prefix="${2:-$default_prefix}"
 
 build_ffmpeg() {
     local arch="$1"