Coin, FFmpeg, iOS: Add output parameter to 'install-ffmpeg-ios.sh'

This patch adds an optional parameter to the provisioning script
'install-ffmpeg-ios.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: 6.9 6.8
Change-Id: Ib0764eb29b84ae8badcfb48424f86316f3b2b650
Reviewed-by: Simo Fält <simo.falt@qt.io>
(cherry picked from commit 0e5937c8a4)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

coin/provisioning/common/unix/install-ffmpeg-ios.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 will build and install FFmpeg static libs
+# Can take an optional final parameter to control installation directory
 set -ex
 
 # Must match or be lower than the minimum iOS version supported by the version of Qt that is
@@ -13,7 +14,8 @@ source "${BASH_SOURCE%/*}/../unix/ffmpeg-installation-utils.sh"
 
 ffmpeg_source_dir=$(download_ffmpeg)
 ffmpeg_config_options=$(get_ffmpeg_config_options "shared")
-prefix="/usr/local/ios/ffmpeg"
+default_prefix="/usr/local/ios/ffmpeg"
+prefix="${1:-$default_prefix}"
 dylib_regex="^@rpath/.*\.dylib$"
 
 build_ffmpeg_ios() {