improve CI build times

[vonovak]

Feature request

It would be nice to get the CI to work faster. This can probably be done by caching the pod_install step and maybe yarn install step.

Possible implementation

We rely on https://github.com/react-native-community/react-native-circleci-orb so potential changes should be done there. We can test them out in this repo and then ship them there.

yarn install is already provided there, with caching (https://github.com/react-native-community/react-native-circleci-orb/blob/master/src/commands/yarn_install.yml) but it throws errors for us if we use it. The error is described in https://github.com/react-native-community/datetimepicker/blob/master/.circleci/config.yml. Issue for this: react-native-community/react-native-circleci-orb#66

the pod_install step could be cached, inspiration can be taken in https://github.com/wordpress-mobile/circleci-orbs or react-native-community/react-native-circleci-orb#58

Code sample

[vonovak]

10 minutes on each platform now, including tests :)