QT_ROOT_DIR instead of Qt5_Dir #96

Job	Run time
build (macos-latest, mac, 5.15.2)	3m 17s
build (windows-latest, windows, 5.12.12, 73)	2m 26s
build (ubuntu-latest, linux, 6.2.*, qtmultimedia)	1m 30s
build (ubuntu-latest, linux, 5.12.12)	1m 56s
	9m 9s

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