Stepper braces for Seckit Cube

I originally had the BTT s42c, closed-loop steppers and, frankly, they were terrible; little documentation, surface artifacts, and noisy.

Swapped them for some TMC 2209's and 0.9* steppers and all those problems went away.

However, there's no good way to tension the belts and using the stepper mounts to do this left them torqued and rotated. These braces push the steppers against the ends of the screw slots and fixes them much more firmly against the frame.