CI: Pin to ffmpeg release 6.1 for Windows CI #247

Job	Run time
build-linux (gcc-9)	5m 14s
build-linux-Qt6 (gcc-9)	6m 34s
build-macos-brew (clang-9)	5m 19s
build-macos-brew-Qt6 (clang-9)	5m 12s
build-macos-no-brew (clang-9)	8m 31s
build-macos-no-brew-Qt6 (clang-9)	10m 28s
build-windows-mingw (5.15.2)	15m 3s
build-windows-mingw-Qt6 (6.3.0, qtmultimedia qt5compat)	17m 44s
build-windows-vs (5.15.2)	11m 42s
build-windows-vs-Qt6 (6.3.0, qtmultimedia qt5compat)	18m 20s
	1h 44m 7s

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