Msys2: Qt5/Qt6 specific .nsi creation scripts
Currently msys2 installer build system has just one qt-client .nsi script creation script, and that's specific to Qt5. Rename it to a 'qt5' one, and add separate one for 'qt6'.
Currently msys2 installer build system has just one qt-client .nsi script creation script, and that's specific to Qt5. Rename it to a 'qt5' one, and add separate one for 'qt6'.