Find MjScene at OnEnable: ensure MuJoCo compilation happens only once at startup.

PiperOrigin-RevId: 498655077
Change-Id: Ibed8f36a880426063b7729fc66b15c67adcc9a65

unity/Runtime/Components/Joints/MjBaseJoint.cs

@@ -49,8 +49,7 @@ namespace Mujoco {
       DofAddress = model->jnt_dofadr[MujocoId];
     }
 
-    protected override void Start() {
-      base.Start();
+    protected void Start() {
       MjBaseBody grandparentBody, parentBody;
       GetConnectedBodies(out grandparentBody, out parentBody);
       GrandParentBody = grandparentBody;

unity/Runtime/Components/MjComponent.cs

@@ -77,7 +77,7 @@ public abstract class MjComponent : MonoBehaviour {
 
   private bool _sceneExcludesMe = false;
 
-  protected unsafe virtual void Start() {
+  protected unsafe virtual void OnEnable() {
     if (MjScene.Instance == null) {
       throw new Exception("MuJoCo Scene not found");
     }

unity/Runtime/Components/MjScene.cs

@@ -85,6 +85,7 @@ public class MjScene : MonoBehaviour {
   public EventHandler<MjStepArgs> preDestroyEvent;
 
   protected unsafe void Start() {
+    SceneRecreationAtLateUpdateRequested = false;
     CreateScene();
   }