Removal of warning spam when using Camera.Render (illegal accessing of cameraColorTarget)

[lordubbe]

This PR moves the accessing of cameraColorTarget to OnCameraSetup from AddRenderPasses to get rid of warning spam when using Camera.Render in another object.

Not sure when this became necessary, but it turns out the fix was relatively simple, so I thought I'd share. I found out about this from this forum post: https://forum.unity.com/threads/azure-sky-dynamic-skybox.322773/page-27#post-6841583