Moved SkyManager Build() to the HDRenderPipeline constructor to avoid leaking Materials each frame.

Assets/ScriptableRenderPipeline/HDRenderPipeline/HDRenderPipeline.cs

             if(m_LightLoop != null)
                 m_LightLoop.Build(owner.textureSettings);
 
+            m_SkyManager.Build();
             m_SkyManager.skyParameters = owner.skyParametersToUse;
         }
 
 
             GraphicsSettings.lightsUseLinearIntensity = true;
             GraphicsSettings.lightsUseColorTemperature = true;
-
-            m_SkyManager.Build();
 
             if (!m_LitRenderLoop.isInit)
                 m_LitRenderLoop.RenderInit(renderContext);

Assets/ScriptableRenderPipeline/HDRenderPipeline/Sky/SkyManager.cs

                 m_UpdateRequired = true;
 
                 if (value != null)
+                {
+                    m_Renderer.Build();
+                }
             }
             get { return m_SkyParameters; }
         }
 
         public void Build()
         {
-            if (m_Renderer != null)
-                m_Renderer.Build();
-
             // Create unititialized. Lazy initialization is performed later.
             m_iblFilterGgx = new IBLFilterGGX();