Allow PARALLEL_PROCESSOR_COUNT to override nprocessors when setting number_of_workers

[GoodForOneFare]

Tapioca currently factors Etc.nprocessors into its worker count calculation: https://github.com/Shopify/tapioca/blob/703ce223ef95b7252480913296881bec7860e221/lib/tapioca/executor.rb#L17-L20

However, the parallel gem currently prioritizes ENV['PARALLEL_PROCESSOR_COUNT'] above nprocessors. This affects environments like Spin, where most gems pick up the processor count and limit their process usage accordingly.