Disable web applet and warning when compiling for Linux on CI

yuzu's web applet does not or barely reacts to user input while open in
Linux. It can be closed via 'Exit Web Applet' on the menubar, however if
yuzu is in fullscreen, this is effectively a softlock as the menubar
cannot be accessed.

This disables building yuzu with the web applet on the Linux CI target.
In addition, this disables the QMessageBox warning about not having
compiled yuzu with the web applet.