> (system (string-append (assoc-ref inputs "xorg-server") > - "/bin/Xvfb :1 &")) > + "/bin/Xvfb :1 -screen 0 640x480x24 &"))
Might (setenv "QT_QPA_PLATFORM" "offscreen") also be enough? Starting an Xvfb server and then not stopping it again is kinda ... extreme. Also, who says that :1 is free? I can see that it was there before, but just to be sure...