VTK disabled on AppImage, Mac, Windows

For now, I’m going to disable VTK on the GitHub nightly builds.

While we now have support to compile against VTK9, nothing actually works because of changes between the VTK 8 and VTK 9 interfaces.

Consequently, we’re working on some contingency plans, particularly for charts / plots.

In the meantime, I’m going to turn off VTK builds so anyone testing Mac and Windows (and AppImage) gets a better experience.

@kevinsmia1939 @RizzerOnGitHub - I would advocate that 1.97 builds (e.g., Flatpak) also turn off VTK features.