example-configs: add xfwm4 component for Xfce 4.20 too

Fedora 42 has Xfce 4.20, build that too. But add it as a separate
component, because at least Debian 12, and also dom0 (F41) have Xfce
4.18.

QubesOS/qubes-issues#9807

example-configs/qubes-os-r4.3.yml

@@ -138,6 +138,8 @@ components:
   - desktop-linux-kde
   - desktop-linux-xfce4
   - desktop-linux-xfce4-xfwm4
+  - desktop-linux-xfce4-xfwm4-420:
+      branch: xfce-4.20
   - desktop-linux-i3
   - desktop-linux-i3-settings-qubes
   - desktop-linux-awesome