cubemap_uri not working ?

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Hello,

I tried adding a cubemap to my scene but it doesn't work. The default sky cubemap is displayed instead. I'm using Ignition Gazebo, version 6.11.0. Did I make a mistake ? here is my <scene> section:

<scene>
  <ambient>0.4 0.4 0.4 1</ambient>
  <background>0.7 0.7 0.7 1</background>
  <shadows>true</shadows>
  <grid>false</grid>
  <sky>
    <time>3</time>
    <sunrise>4</sunrise>
    <sunset>21</sunset>
    <clouds>
      <speed>1.2</speed>
      <direction>1.5</direction>
      <mean_size>0.2</mean_size>
      <humidity>0.9</humidity>
      <ambient>0.1 0.2 0.3</ambient>
    </clouds>
    <cubemap_uri>./textures/cubemap.dds</cubemap_uri>
  </sky>

</scene>