Fix issue #3665

[CMDisme]

Set cubemap format in LoadTextureCubemap

[CMDisme]

Fixes #3665

[raysan5]

@CMDisme how did you test this change if there is no code example to test it?

[CMDisme]

I copied the code from the issue

[raysan5]

@CMDisme Ok, I see, but I'm afraid that code does not verify if the loaded cubemap is actually valid. And example would be required to review it.