DolphinQt: Properly define QtPaths

[JoshuaVandaele]

This fixes cross-compilation on Windows, where windeplotqt would previously always grab the host's DLLs instead of the target's.

Before/after:

$ file Dolphin-arm64/*.dll
Dolphin-arm64/Qt6Core.dll:    PE32+ executable for MS Windows 6.00 (DLL), x86-64, 7 sections
Dolphin-arm64/Qt6Gui.dll:     PE32+ executable for MS Windows 6.00 (DLL), x86-64, 6 sections
Dolphin-arm64/Qt6Svg.dll:     PE32+ executable for MS Windows 6.00 (DLL), x86-64, 6 sections
Dolphin-arm64/Qt6Widgets.dll: PE32+ executable for MS Windows 6.00 (DLL), x86-64, 6 sections

$ file Dolphin-arm64/*.dll
Dolphin-arm64/Qt6Core.dll:    PE32+ executable for MS Windows 6.02 (DLL), ARM64, 7 sections
Dolphin-arm64/Qt6Gui.dll:     PE32+ executable for MS Windows 6.02 (DLL), ARM64, 6 sections
Dolphin-arm64/Qt6Svg.dll:     PE32+ executable for MS Windows 6.02 (DLL), ARM64, 6 sections
Dolphin-arm64/Qt6Widgets.dll: PE32+ executable for MS Windows 6.02 (DLL), ARM64, 6 sections