[Development] Enabling xcb-native-painting feature in CI

[Albert Astals Cid]

Can we have CI build with this flag enabled by default?

A recent commit broke the qtbase build

src/plugins/platforms/xcb/nativepainting/qpixmap_x11.cpp:1337:21: error: use 
of deleted function ‘QX11PlatformPixmap& QX11PlatformPixmap::operator=(const 
QX11PlatformPixmap&)’
 1337 |             *this = newData;

It is my undestading that it being enabled does nothing unless you also 
specify the QT_XCB_NATIVE_PAINTING environment variable, so it seems like it 
would be a good thing to cover a bit more of cases in CI.

Cheers,
  Albert