Improve names of client buffer integration variable names

QT_WAYLAND_CLIENT_BUFFER_INTEGRATION env variable is used by both compositor and client, but it means different thing for both.

E.g., on RPI, you need to set

QT_WAYLAND_CLIENT_BUFFER_INTEGRATION=linux-dmabuf-unstable-v1  

for compositor to work.

But this env cannot be used when launching clients, as they will fail with

qt.qpa.wayland: Failed to load client buffer integration: "linux-dmabuf-unstable-v1"
qt.qpa.wayland: Available client buffer integrations: QList("wayland-egl") 

This makes it hard to create environment where you can test both wayland compositor and clients.