[CI] Super naive iOS build on every commit

Summary:
Basically taking what we currently have for releases, but skipping the export/upload step.

This doesn't have any caching, doesn't care if irrelevant directories have changed, etc. Just *something* before we close automation work for the time being.

Test Plan: Worked on fork

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: palys-swm, Adrian, karol-bisztyga, benschac, boristopalov

Differential Revision: https://phabricator.ashoat.com/D2418