infra: add default branch name to outputs

.gitea/workflows/gentoo-utils.yml

@@ -20,6 +20,7 @@ jobs:
       - name: Check for changes before building
         id: image-changes
         run: |
+          echo "branch_name=${GITHUB_HEAD_REF:-${GITHUB_REF#refs/heads/}}" >> $GITEA_OUTPUT
           if ! git diff ${{ gitea.event.before }} ${{ gitea.sha }} --no-patch --exit-code .docker; then
             echo changes_detected=true >> $GITEA_OUTPUT
           else