Resolve deprecation warning for `pytensor`'s `Variable`

[RyanAugust]

Pytensor has renamed pytensor.tensor.var to pytensor.tensor.variable. Imports in pymc_bart rely on the legacy .var call, which within pytensor resolves by throwing a warning and importing the renamed variable sub-module. Pytensor commit ref

This PR simply updates pymc_bart to call the modern .variable directly

[aloctavodia]

Thanks @RyanAugust!