Fix GITHUB_OUTPUT variable expansion to prevent empty comment body

Co-authored-by: commjoen <[email protected]>

Author: Copilot

.github/workflows/preview.yml

@@ -200,9 +200,9 @@ jobs:
           EOF
           # yamllint enable rule:line-length
 
-          echo "content<<EOF" >> \$GITHUB_OUTPUT
-          cat instructions.md >> \$GITHUB_OUTPUT
-          echo "EOF" >> \$GITHUB_OUTPUT
+          echo "content<<EOF" >> $GITHUB_OUTPUT
+          cat instructions.md >> $GITHUB_OUTPUT
+          echo "EOF" >> $GITHUB_OUTPUT
 
       - name: Comment on PR
         uses: actions/github-script@v7