qt510: disable framework build on macOS

We prefer to have $out/include & $out/lib installed so that things
like multiple outputs can work. There is no way to do this in Qt
currently without also disabling the ‘framework’ install. Not sure if
this will break anything but it will fix issues we run into on some
projects like python2.7-pyside:

Linux: https://hydra.nixos.org/build/73219181
macOS: https://hydra.nixos.org/build/73059550

Hopefully this gives us consistency between the two.