`predict()` parallelization breaks with terra (external pointer invalid)

[fdetsch]

Parallelization in predict() via the parallel 📦 triggered the following error, and was temporarily removed in 9ae25cc.

Error in h():
! error in evaluating the argument 'x' in selecting a method for function 'rast': 4 nodes produced errors; first error: error in evaluating the argument 'x' in selecting a method for function 'app': external pointer is not valid

The future ecosystem might handle terra 📦 objects more gracefully across parallel backends and may be a suitable drop-in replacement.