ci: allow branch-dispatched Fern publish tests

Author: andreatgretel

.github/workflows/build-fern-docs.yml

@@ -85,7 +85,7 @@ jobs:
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6
         with:
           path: workflow
-          ref: ${{ github.event.repository.default_branch }}
+          ref: ${{ github.event_name == 'workflow_dispatch' && github.ref || github.event.repository.default_branch }}
 
       - name: Checkout source
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6