CI: make the tag step idempotent on force rebuilds

Force-rebuilding an already-published version made the final tag push fail
("tag already exists"), turning an otherwise successful run red. Skip
tagging when the tag already exists on the remote.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/build-flatpak.yml

@@ -142,5 +142,10 @@ jobs:
           TOKEN: ${{ secrets.PUBLISH_TOKEN != '' && secrets.PUBLISH_TOKEN || secrets.GITHUB_TOKEN }}
         run: |
           AUTH_URL="$(echo "${{ github.server_url }}" | sed "s#://#://${{ github.actor }}:${TOKEN}@#")/${{ github.repository }}.git"
-          git tag "v${{ steps.ver.outputs.version }}"
-          git push "$AUTH_URL" "v${{ steps.ver.outputs.version }}"
+          TAG="v${{ steps.ver.outputs.version }}"
+          if git ls-remote --tags "$AUTH_URL" "refs/tags/$TAG" | grep -q .; then
+            echo "Tag $TAG already exists (force rebuild) - leaving it in place."
+          else
+            git tag "$TAG"
+            git push "$AUTH_URL" "$TAG"
+          fi