[JENKINS-32765] Perhaps it makes more sense to make this a property of the PluginManager rather than the classic strategy

- Also expose via servlet context parameter of same name to allow multiple instances of Jenkins in the same servlet container to have different directories
- As we are no longer a constant, change the system property to camelCase

core/src/main/java/hudson/ClassicPluginStrategy.java

@@ -88,12 +88,6 @@ public class ClassicPluginStrategy implements PluginStrategy {
         }
     };
 
-    /**
-     * If set this should be the absolute file path to the the base directory for all exploded .hpi/.jpi plugins.
-     * If {@literal null} then plugins will be exploded into {@literal JENKINS_HOME/plugins}.
-     */
-    private static final String WORK_DIR = System.getProperty(ClassicPluginStrategy.class.getName() + ".WORK_DIR");
-
     private PluginManager pluginManager;
 
     /**
@@ -168,10 +162,8 @@ public class ClassicPluginStrategy implements PluginStrategy {
             if (archive.isDirectory()) {// already expanded
                 expandDir = archive;
             } else {
-                expandDir = WORK_DIR == null
-                        ? new File(archive.getParentFile(), getBaseName(archive.getName()))
-                        : new File(WORK_DIR, getBaseName(archive.getName()));
-                ;
+                File f = pluginManager.getWorkDir();
+                expandDir =  new File(f == null ? archive.getParentFile() : f, getBaseName(archive.getName()));
                 explode(archive, expandDir);
             }