`DatacubeExtension.variables` doesn't have a setter

[TomAugspurger]

There's no variables.setter defined at

pystac/pystac/extensions/datacube.py

Line 548 in 6e6e987

def variables(self) -> Optional[Dict[str, Variable]]:

.

I think there should be one, analogous to

pystac/pystac/extensions/datacube.py

Lines 543 to 545 in 6e6e987

	@dimensions.setter
	def dimensions(self, v: Dict[str, Dimension]) -> None:
	self._set_property(DIMENSIONS_PROP, {k: dim.to_dict() for k, dim in v.items()})

.