CI: Pin to ffmpeg release 6.1 for Windows CI #245

Job	Run time
build-macos-brew (clang-9)	4m 12s
build-linux (gcc-9)	5m 55s
build-linux-Qt6 (gcc-9)	6m 18s
build-macos-brew-Qt6 (clang-9)	5m 55s
build-macos-no-brew (clang-9)	9m 34s
build-macos-no-brew-Qt6 (clang-9)	13m 19s
build-windows-mingw (5.15.2)	2m 19s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	2m 29s
build-windows-vs (5.15.2)	2m 34s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	3m 28s
	56m 3s

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