ci: fix syntax error

Author: Genyus

.github/workflows/classroom-autograding.yml

@@ -74,7 +74,7 @@ jobs:
             const passed = parseInt(${{ steps.parse.outputs.passed || 0 }}, 10);
             const failed = parseInt(${{ steps.parse.outputs.failed || 0 }}, 10);
             const skipped = parseInt(${{ steps.parse.outputs.skipped || 0 }}, 10);
-            const total = ${env.MAX_TESTS};
+            const total = ${{ env.MAX_TESTS }};
             const result = `${passed}/${total}`
 
             core.setOutput('total', total);