[CI] Make premerge jobs support GHA postcommit (#176180)

This was causing failures in the release branch as the premerge jobs
there are also run postcommit through GHA. We were expecting a PR number
to always be present when it was not.

.ci/utils.sh

@@ -37,11 +37,15 @@ function at-exit {
     python "${MONOREPO_ROOT}"/.ci/generate_test_report_github.py \
       $retcode "${BUILD_DIR}"/test-results.*.xml "${MONOREPO_ROOT}"/ninja*.log \
       >> $GITHUB_STEP_SUMMARY
+    if [[ -n "$GITHUB_PR_NUMBER" ]]; then
       (python "${MONOREPO_ROOT}"/.ci/premerge_advisor_explain.py \
         $(git rev-parse HEAD~1) $retcode "${GITHUB_TOKEN}" \
         $GITHUB_PR_NUMBER "${BUILD_DIR}"/test-results.*.xml \
         "${MONOREPO_ROOT}"/ninja*.log)
       advisor_retcode=$?
+    else
+      advisor_retcode=$retcode
+    fi
   fi
 
   if [[ "$retcode" != "0" ]]; then