Generate report.xml report with --build_only too

tools/run_tests/run_tests.py

@@ -1059,13 +1059,15 @@ def _build_and_run(
     check_cancelled, newline_on_success, cache, xml_report=None, build_only=False):
   """Do one pass of building & running tests."""
   # build latest sequentially
-  num_failures, _ = jobset.run(
+  num_failures, resultset = jobset.run(
       build_steps, maxjobs=1, stop_on_failure=True,
       newline_on_success=newline_on_success, travis=args.travis)
   if num_failures:
     return [BuildAndRunError.BUILD]
 
   if build_only:
+    if xml_report:
+      report_utils.render_junit_xml_report(resultset, xml_report)
     return []
 
   # start antagonists