Actions: switch back to upstream Flatpak Builder action (#51)

See: flathub-infra/flatpak-github-actions#17 (comment)

Author: cassidyjames

.github/workflows/flatpak.yml

@@ -18,7 +18,7 @@ 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: tally.flatpak
         manifest-path: build-aux/flatpak/com.cassidyjames.plausible.json