live: try to mount flatpak before the volatile overlay

what I want to happen is that the flatpak is also overlayed though I am
not convinced that works the way I think it does

mkosi.extra/usr/lib/systemd/system-generators/kde-linux-live-generator

@@ -44,6 +44,7 @@ cat <<- EOF > "$normal_dir/var-lib-flatpak.mount"
 # Generated by $(basename "$0")
 [Unit]
 Description=Mount unit for /var/lib/flatpak
+Before=systemd-volatile-root.service
 
 [Mount]
 What=/dev/disk/by-label/KDEOSLive