BazelUtils.showFailedActions: correctly convert

Correctly cast events to string (the introduction of += in
the code review of the last change to that method removed
the implicit conversion).

Change-Id: Ie7fca226941c15a3b2fea9a90b2a7b7d8dce4d66
diff --git a/jenkins/lib/src/build/bazel/ci/BazelUtils.groovy b/jenkins/lib/src/build/bazel/ci/BazelUtils.groovy
index 43029fd..430f88f 100644
--- a/jenkins/lib/src/build/bazel/ci/BazelUtils.groovy
+++ b/jenkins/lib/src/build/bazel/ci/BazelUtils.groovy
@@ -113,7 +113,7 @@
     def eventsstring = ""
     for(event in events) {
       if ("action" in event) {
-        eventsstring += event + "\n"
+        eventsstring += event.toString() + "\n"
       }
     }
     if (eventsstring == "") {