Msys2 Makefile.meson: Qt6-client installer
Add Qt6-client installer build to Msys2 Makefile.meson. Only for ucrt64 target (as Qt6 requires win10 anyway)
Requires the new msys2 environment: #45966
Add Qt6-client installer build to Msys2 Makefile.meson. Only for ucrt64 target (as Qt6 requires win10 anyway)