ci: remove extra array wrapping and output empty manager_list as valid JSON

Signed-off-by: Ahmed Al-Nassif <mr.ahmed.nassif@gmail.com>

.github/workflows/main.yml

@@ -394,10 +394,10 @@ jobs:
             *) echo "Skipping manager resolution (no root flavor / none)." ;;
           esac
           if [ -n "$LIST" ]; then
-            echo "manager_list=[$LIST]" >> "$GITHUB_OUTPUT"
+            echo "manager_list=$LIST" >> "$GITHUB_OUTPUT"
             echo "Manager sources resolved."
           else
-            echo "manager_list=" >> "$GITHUB_OUTPUT"
+            echo "manager_list=[]" >> "$GITHUB_OUTPUT"
           fi
 
       - name: Compose requested feature set