Flatpak: use master runtime of GNOME sdk

.gitlab-ci.yml

@@ -55,7 +55,7 @@ variables:
         - web
 
 flatpak:
-    image: registry.gitlab.gnome.org/gnome/gnome-runtime-images/rust_bundle:3.28
+    image: registry.gitlab.gnome.org/gnome/gnome-runtime-images/rust_bundle:master
     stage: test
     script:
         - flatpak-builder --stop-at=gnome-podcasts app org.gnome.Podcasts.json

org.gnome.Podcasts.json

@@ -1,7 +1,7 @@
 {
     "app-id" : "org.gnome.Podcasts",
     "runtime" : "org.gnome.Platform",
-    "runtime-version" : "3.28",
+    "runtime-version" : "master",
     "sdk" : "org.gnome.Sdk",
     "sdk-extensions" : [
         "org.freedesktop.Sdk.Extension.rust-stable"