[JENKINS-27329] Less aggressive WorkspaceCleanupThread (#3444)

* JENKINS-27329 Less aggressive WorkspaceCleanupThread

I dare to claim that the default behaviour of WorkspaceCleanupThread is too aggressive => this little change is by no means perfect (or admittedly even far from perfect), but IMHO a saner or slightly more defensive default behaviour.

Mind that according to https://github.com/jenkinsci/jenkins/blob/9e64bcdcb4a2cf12d59dfa334e09ffb448d361e9/core/src/main/java/hudson/model/Job.java#L301 this "only" checks whether or not the last build of a job is in progress, while the JavaDoc says "Returns true if a build of this project is in progress." (cf. http://javadoc.jenkins-ci.org/hudson/model/Job.html#isBuilding--)

* Fix compilation

* Dummy commit to trigger pipeline

Previous pipeline execution (https://ci.jenkins.io/blue/organizations/jenkins/Core%2Fjenkins/detail/PR-3444/2/tests) failed with one failing test that at first glance appears to be unrelated with my change(s) and looks like a flaky test?

* Add fine logging message

core/src/main/java/hudson/model/WorkspaceCleanupThread.java

@@ -139,6 +139,15 @@ public class WorkspaceCleanupThread extends AsyncPeriodicWork {
             }
         }
 
+        // TODO this may only check the last build in fact:
+        if (item instanceof Job<?,?>) {
+            Job<?,?> j = (Job<?,?>) item;
+            if (j.isBuilding()) {
+                LOGGER.log(Level.FINE, "Job {0} is building, so not deleting", item.getFullDisplayName());
+                return false;
+            }
+        }
+
         LOGGER.log(Level.FINER, "Going to delete directory {0}", dir);
         return true;
     }