flatpak: Build against the master runtime

This is what the ci is set up to use, and building
against 3.34 fails there.

build-aux/flatpak/org.gtk.Demo4.json

@@ -1,7 +1,7 @@
 {
     "app-id": "org.gtk.Demo4",
     "runtime": "org.gnome.Platform",
-    "runtime-version": "3.34",
+    "runtime-version": "master",
     "sdk": "org.gnome.Sdk",
     "command": "gtk4-demo",
     "tags": ["devel", "development", "nightly"],