Workflow updates for screenshot

Author: slaclau

.github/workflows/gui.yaml

@@ -31,7 +31,7 @@ jobs:
         run: |
           export XDG_RUNTIME_DIR=$(mktemp -d)
           eval $(dbus-launch --auto-syntax)
-          dbus-run-session sh
+          dbus-run-session gnome-session
           mutter --wayland --no-x11 --sm-disable --headless -- python test/test_demo.py
       - name: Upload screenshot
         uses: actions/upload-artifact@v4