QT_ROOT_DIR instead of Qt5_Dir #100

Job	Run time
build (ubuntu-latest, linux, 6.2.*, qtmultimedia)	1m 14s
build (windows-latest, windows, 5.12.12, 73)	2m 38s
build (ubuntu-latest, linux, 5.12.12)	1m 18s
build (macos-latest, mac, 5.15.2)	3m 11s
	8m 21s

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