tooling: build gfm for summer 2025 (_s25 branch)

Author: cagix

.github/workflows/gfm.yaml

@@ -24,6 +24,6 @@ jobs:
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           publish_dir: _gfm
-          publish_branch: _gfm
+          publish_branch: _s25    ## TODO (semester branch)
           force_orphan: true
           commit_message: ${{ github.event.head_commit.message }}