CI: Use version 4 of the flatpak-builder action

None of the changes included in v4 affect OBS Studio, but since
we're already touching this code, let's make sure it's updated.

.github/workflows/flatpak.yml

@@ -47,7 +47,7 @@ jobs:
           submodules: 'recursive'
 
       - name: Build Flatpak Manifest
-        uses: bilelmoussaoui/flatpak-github-actions/flatpak-builder@v3
+        uses: bilelmoussaoui/flatpak-github-actions/flatpak-builder@v4
         if: success() && (github.event_name != 'pull_request' || env.SEEKING_TESTERS == '1')
         with:
           bundle: obs-studio-${{ github.sha }}.flatpak