Workflows: Switch to upstream Flatpak Builder (#30)

See: flathub-infra/flatpak-github-actions#17

Author: cassidyjames

.github/workflows/flatpak.yml

@@ -17,7 +17,7 @@ jobs:
       options: --privileged
     steps:
     - uses: actions/checkout@v4
-    - uses: flathub-infra/flatpak-github-actions/flatpak-builder@master
+    - uses: flatpak/flatpak-github-actions/flatpak-builder@v6
       with:
         bundle: rota.flatpak
         manifest-path: linux/net.hhoney.rota.yml