Compressed textures aren't allowed as cubemap in GL4.1 and earlier.

[cwfitzgerald]

I'm not actually sure why this is a rule, but on nv on a 4.1 context, I'm getting GLES: [API/Error] ID 1280 : GL_INVALID_ENUM error generated. Texture type and format combination is not valid.

[inner-daemons]

I also uncovered this in #8670 on CI, which uses LLVMPIPE.