Reflection Probe Cubemap Captures Show Black Sky on ProceduralSkyMaterial

[DoubleStyx]

Summary
Because of how the ProceduralSky uses a sphere mesh for its vertex interpolation, there is odd behavior around cubemap baking where only the top face is actually colored blue and where the lateral faces are black.

Reproduction Steps

1. Open a default new world.
2. Create a reflection probe.
3. Observe black skybox colors on the lateral faces.

Expected / actual

Expected: Normal reflection probe capture.

Actual: Black artifacts.

Logs / attachments

[Baplar]

This looks a lot like what was fixed a while ago with 38ef3a4
Also the same behavior seems to happen in cameras

[Baplar]

This is fixed by #622