CI: Use flatpak-builder subaction

The Flatpak action now contains two subactions:

- flatpak-builder: for building and uploading a bundle
- flat-manager: for deploying the bundle to a remote repository

Use the right action (flatpak-builder) for the Flatpak workflow.
This won't affect existing pull requests, except the ones that
have the "Seeking Testers" label applied - in which case, they
simply need to rebase against the master branch.

.github/workflows/flatpak.yml

@@ -35,7 +35,7 @@ jobs:
         submodules: 'recursive'
 
     - name: Build Flatpak Manifest
-      uses: bilelmoussaoui/flatpak-github-actions@master
+      uses: bilelmoussaoui/flatpak-github-actions/flatpak-builder@master
       if: success() && (github.event_name != 'pull_request' || env.SEEKING_TESTERS == '1')
       with:
         bundle: obs-studio-${{ github.sha }}.flatpak