CI: Slim down Linux package install list

scripts/appimage/make-appimage.sh

@@ -109,6 +109,7 @@ done
 echo "Running linuxdeploy to create AppDir..."
 EXTRA_QT_PLUGINS="core;gui;network;svg;waylandclient;widgets;xcbqpa" \
 EXTRA_PLATFORM_PLUGINS="libqwayland-egl.so;libqwayland-generic.so" \
+DEPLOY_PLATFORM_THEMES="1" \
 QMAKE="$DEPSDIR/bin/qmake" \
 NO_STRIP="1" \
 $LINUXDEPLOY --plugin qt --appdir="$OUTDIR" --executable="$BUILDDIR/bin/duckstation-qt" \