Add Cocoapods as step in prepare dependencies and use run_if to control if it's being run

[dominik-hadl]

This will allow us to properly use Bitrise caching and further simplify the Fastfiles.

See the example of how run_if works here:

https://github.com/bitrise-io/bitrise/blob/master/_examples/experimentals/templates/bitrise.yml

And related issue: https://discuss.bitrise.io/t/allow-disabling-a-step-in-the-workflow/711/5