Can't seem to use multiple cores to achieve speedups in Generate Morphs GitHub Action

[stefmolin]

On local machines, both the multiprocessing logic introduced in https://github.com/stefmolin/data-morph/pull/273 and the GNU parallel command (https://github.com/stefmolin/data-morph/pull/222) provide significant speedups over running serially. However, this doesn't seem to work when running in GitHub runners.