CI: Don't publish betas to Flathub stable

Beta releases are being considered, in which case the tag
name will contain '-beta' instead of '-rc'. Adapt the CI
workflow to take '-beta' into account too.

.github/workflows/flatpak.yml

@@ -107,7 +107,7 @@ jobs:
 
       - name: "Publish to Flathub"
         uses: bilelmoussaoui/flatpak-github-actions/flat-manager@v4
-        if: "!contains(github.ref, '-rc')"
+        if: "!contains(github.ref, '-beta') && !contains(github.ref, '-rc')"
         with:
           flat-manager-url: https://hub.flathub.org/
           repository: stable