[JENKINS-60867] Prevent creation of duplicated SetupWizard singleton instances on Jenkins startup (#4456)

* [JENKINS-60867] Blank page after SetupWizard: root cause

* Correct a test that was a bit too greedy

core/src/main/java/jenkins/model/Jenkins.java

@@ -3292,7 +3292,7 @@ public class Jenkins extends AbstractCIBase implements DirectlyModifiableTopLeve
                 for (Action a : getExtensionList(RootAction.class))
                     if (!actions.contains(a)) actions.add(a);
 
-                setupWizard = new SetupWizard();
+                setupWizard = ExtensionList.lookupSingleton(SetupWizard.class);
                 getInstallState().initializeState();
             }
         });