fix the issue with summarize-checks not using impact assessment target branch when it is available (#36601)

Author: scbedd

.github/workflows/src/summarize-checks/summarize-checks.js

@@ -386,6 +386,7 @@ export async function summarizeChecksImpl(
 
   if (impactAssessment) {
     core.info(`ImpactAssessment: ${JSON.stringify(impactAssessment)}`);
+    targetBranch = impactAssessment.targetBranch;
   } else {
     core.info(
       `No impact assessment found for ${owner}/${repo}#${issue_number}. ` +